5 files changed, 1003 insertions, 17 deletions

diff --git a/Documentation/ww-mutex-design.txt b/Documentation/ww-mutex-design.txt
new file mode 100644
index 000000000000..8a112dc304c3
--- /dev/null
+++ b/Documentation/ww-mutex-design.txt
@@ -0,0 +1,344 @@
+Wait/Wound Deadlock-Proof Mutex Design
+======================================
+
+Please read mutex-design.txt first, as it applies to wait/wound mutexes too.
+
+Motivation for WW-Mutexes
+-------------------------
+
+GPU's do operations that commonly involve many buffers.  Those buffers
+can be shared across contexts/processes, exist in different memory
+domains (for example VRAM vs system memory), and so on.  And with
+PRIME / dmabuf, they can even be shared across devices.  So there are
+a handful of situations where the driver needs to wait for buffers to
+become ready.  If you think about this in terms of waiting on a buffer
+mutex for it to become available, this presents a problem because
+there is no way to guarantee that buffers appear in a execbuf/batch in
+the same order in all contexts.  That is directly under control of
+userspace, and a result of the sequence of GL calls that an application
+makes.  Which results in the potential for deadlock.  The problem gets
+more complex when you consider that the kernel may need to migrate the
+buffer(s) into VRAM before the GPU operates on the buffer(s), which
+may in turn require evicting some other buffers (and you don't want to
+evict other buffers which are already queued up to the GPU), but for a
+simplified understanding of the problem you can ignore this.
+
+The algorithm that the TTM graphics subsystem came up with for dealing with
+this problem is quite simple.  For each group of buffers (execbuf) that need
+to be locked, the caller would be assigned a unique reservation id/ticket,
+from a global counter.  In case of deadlock while locking all the buffers
+associated with a execbuf, the one with the lowest reservation ticket (i.e.
+the oldest task) wins, and the one with the higher reservation id (i.e. the
+younger task) unlocks all of the buffers that it has already locked, and then
+tries again.
+
+In the RDBMS literature this deadlock handling approach is called wait/wound:
+The older tasks waits until it can acquire the contended lock. The younger tasks
+needs to back off and drop all the locks it is currently holding, i.e. the
+younger task is wounded.
+
+Concepts
+--------
+
+Compared to normal mutexes two additional concepts/objects show up in the lock
+interface for w/w mutexes:
+
+Acquire context: To ensure eventual forward progress it is important the a task
+trying to acquire locks doesn't grab a new reservation id, but keeps the one it
+acquired when starting the lock acquisition. This ticket is stored in the
+acquire context. Furthermore the acquire context keeps track of debugging state
+to catch w/w mutex interface abuse.
+
+W/w class: In contrast to normal mutexes the lock class needs to be explicit for
+w/w mutexes, since it is required to initialize the acquire context.
+
+Furthermore there are three different class of w/w lock acquire functions:
+
+* Normal lock acquisition with a context, using ww_mutex_lock.
+
+* Slowpath lock acquisition on the contending lock, used by the wounded task
+  after having dropped all already acquired locks. These functions have the
+  _slow postfix.
+
+  From a simple semantics point-of-view the _slow functions are not strictly
+  required, since simply calling the normal ww_mutex_lock functions on the
+  contending lock (after having dropped all other already acquired locks) will
+  work correctly. After all if no other ww mutex has been acquired yet there's
+  no deadlock potential and hence the ww_mutex_lock call will block and not
+  prematurely return -EDEADLK. The advantage of the _slow functions is in
+  interface safety:
+  - ww_mutex_lock has a __must_check int return type, whereas ww_mutex_lock_slow
+    has a void return type. Note that since ww mutex code needs loops/retries
+    anyway the __must_check doesn't result in spurious warnings, even though the
+    very first lock operation can never fail.
+  - When full debugging is enabled ww_mutex_lock_slow checks that all acquired
+    ww mutex have been released (preventing deadlocks) and makes sure that we
+    block on the contending lock (preventing spinning through the -EDEADLK
+    slowpath until the contended lock can be acquired).
+
+* Functions to only acquire a single w/w mutex, which results in the exact same
+  semantics as a normal mutex. This is done by calling ww_mutex_lock with a NULL
+  context.
+
+  Again this is not strictly required. But often you only want to acquire a
+  single lock in which case it's pointless to set up an acquire context (and so
+  better to avoid grabbing a deadlock avoidance ticket).
+
+Of course, all the usual variants for handling wake-ups due to signals are also
+provided.
+
+Usage
+-----
+
+Three different ways to acquire locks within the same w/w class. Common
+definitions for methods #1 and #2:
+
+static DEFINE_WW_CLASS(ww_class);
+
+struct obj {
+        struct ww_mutex lock;
+        /* obj data */
+};
+
+struct obj_entry {
+        struct list_head head;
+        struct obj *obj;
+};
+
+Method 1, using a list in execbuf->buffers that's not allowed to be reordered.
+This is useful if a list of required objects is already tracked somewhere.
+Furthermore the lock helper can use propagate the -EALREADY return code back to
+the caller as a signal that an object is twice on the list. This is useful if
+the list is constructed from userspace input and the ABI requires userspace to
+not have duplicate entries (e.g. for a gpu commandbuffer submission ioctl).
+
+int lock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
+{
+        struct obj *res_obj = NULL;
+        struct obj_entry *contended_entry = NULL;
+        struct obj_entry *entry;
+
+        ww_acquire_init(ctx, &ww_class);
+
+retry:
+        list_for_each_entry (entry, list, head) {
+                if (entry->obj == res_obj) {
+                        res_obj = NULL;
+                        continue;
+                }
+                ret = ww_mutex_lock(&entry->obj->lock, ctx);
+                if (ret < 0) {
+                        contended_entry = entry;
+                        goto err;
+                }
+        }
+
+        ww_acquire_done(ctx);
+        return 0;
+
+err:
+        list_for_each_entry_continue_reverse (entry, list, head)
+                ww_mutex_unlock(&entry->obj->lock);
+
+        if (res_obj)
+                ww_mutex_unlock(&res_obj->lock);
+
+        if (ret == -EDEADLK) {
+                /* we lost out in a seqno race, lock and retry.. */
+                ww_mutex_lock_slow(&contended_entry->obj->lock, ctx);
+                res_obj = contended_entry->obj;
+                goto retry;
+        }
+        ww_acquire_fini(ctx);
+
+        return ret;
+}
+
+Method 2, using a list in execbuf->buffers that can be reordered. Same semantics
+of duplicate entry detection using -EALREADY as method 1 above. But the
+list-reordering allows for a bit more idiomatic code.
+
+int lock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
+{
+        struct obj_entry *entry, *entry2;
+
+        ww_acquire_init(ctx, &ww_class);
+
+        list_for_each_entry (entry, list, head) {
+                ret = ww_mutex_lock(&entry->obj->lock, ctx);
+                if (ret < 0) {
+                        entry2 = entry;
+
+                        list_for_each_entry_continue_reverse (entry2, list, head)
+                                ww_mutex_unlock(&entry2->obj->lock);
+
+                        if (ret != -EDEADLK) {
+                                ww_acquire_fini(ctx);
+                                return ret;
+                        }
+
+                        /* we lost out in a seqno race, lock and retry.. */
+                        ww_mutex_lock_slow(&entry->obj->lock, ctx);
+
+                        /*
+                         * Move buf to head of the list, this will point
+                         * buf->next to the first unlocked entry,
+                         * restarting the for loop.
+                         */
+                        list_del(&entry->head);
+                        list_add(&entry->head, list);
+                }
+        }
+
+        ww_acquire_done(ctx);
+        return 0;
+}
+
+Unlocking works the same way for both methods #1 and #2:
+
+void unlock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
+{
+        struct obj_entry *entry;
+
+        list_for_each_entry (entry, list, head)
+                ww_mutex_unlock(&entry->obj->lock);
+
+        ww_acquire_fini(ctx);
+}
+
+Method 3 is useful if the list of objects is constructed ad-hoc and not upfront,
+e.g. when adjusting edges in a graph where each node has its own ww_mutex lock,
+and edges can only be changed when holding the locks of all involved nodes. w/w
+mutexes are a natural fit for such a case for two reasons:
+- They can handle lock-acquisition in any order which allows us to start walking
+  a graph from a starting point and then iteratively discovering new edges and
+  locking down the nodes those edges connect to.
+- Due to the -EALREADY return code signalling that a given objects is already
+  held there's no need for additional book-keeping to break cycles in the graph
+  or keep track off which looks are already held (when using more than one node
+  as a starting point).
+
+Note that this approach differs in two important ways from the above methods:
+- Since the list of objects is dynamically constructed (and might very well be
+  different when retrying due to hitting the -EDEADLK wound condition) there's
+  no need to keep any object on a persistent list when it's not locked. We can
+  therefore move the list_head into the object itself.
+- On the other hand the dynamic object list construction also means that the -EALREADY return
+  code can't be propagated.
+
+Note also that methods #1 and #2 and method #3 can be combined, e.g. to first lock a
+list of starting nodes (passed in from userspace) using one of the above
+methods. And then lock any additional objects affected by the operations using
+method #3 below. The backoff/retry procedure will be a bit more involved, since
+when the dynamic locking step hits -EDEADLK we also need to unlock all the
+objects acquired with the fixed list. But the w/w mutex debug checks will catch
+any interface misuse for these cases.
+
+Also, method 3 can't fail the lock acquisition step since it doesn't return
+-EALREADY. Of course this would be different when using the _interruptible
+variants, but that's outside of the scope of these examples here.
+
+struct obj {
+        struct ww_mutex ww_mutex;
+        struct list_head locked_list;
+};
+
+static DEFINE_WW_CLASS(ww_class);
+
+void __unlock_objs(struct list_head *list)
+{
+        struct obj *entry, *temp;
+
+        list_for_each_entry_safe (entry, temp, list, locked_list) {
+                /* need to do that before unlocking, since only the current lock holder is
+                allowed to use object */
+                list_del(&entry->locked_list);
+                ww_mutex_unlock(entry->ww_mutex)
+        }
+}
+
+void lock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
+{
+        struct obj *obj;
+
+        ww_acquire_init(ctx, &ww_class);
+
+retry:
+        /* re-init loop start state */
+        loop {
+                /* magic code which walks over a graph and decides which objects
+                 * to lock */
+
+                ret = ww_mutex_lock(obj->ww_mutex, ctx);
+                if (ret == -EALREADY) {
+                        /* we have that one already, get to the next object */
+                        continue;
+                }
+                if (ret == -EDEADLK) {
+                        __unlock_objs(list);
+
+                        ww_mutex_lock_slow(obj, ctx);
+                        list_add(&entry->locked_list, list);
+                        goto retry;
+                }
+
+                /* locked a new object, add it to the list */
+                list_add_tail(&entry->locked_list, list);
+        }
+
+        ww_acquire_done(ctx);
+        return 0;
+}
+
+void unlock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
+{
+        __unlock_objs(list);
+        ww_acquire_fini(ctx);
+}
+
+Method 4: Only lock one single objects. In that case deadlock detection and
+prevention is obviously overkill, since with grabbing just one lock you can't
+produce a deadlock within just one class. To simplify this case the w/w mutex
+api can be used with a NULL context.
+
+Implementation Details
+----------------------
+
+Design:
+  ww_mutex currently encapsulates a struct mutex, this means no extra overhead for
+  normal mutex locks, which are far more common. As such there is only a small
+  increase in code size if wait/wound mutexes are not used.
+
+  In general, not much contention is expected. The locks are typically used to
+  serialize access to resources for devices. The only way to make wakeups
+  smarter would be at the cost of adding a field to struct mutex_waiter. This
+  would add overhead to all cases where normal mutexes are used, and
+  ww_mutexes are generally less performance sensitive.
+
+Lockdep:
+  Special care has been taken to warn for as many cases of api abuse
+  as possible. Some common api abuses will be caught with
+  CONFIG_DEBUG_MUTEXES, but CONFIG_PROVE_LOCKING is recommended.
+
+  Some of the errors which will be warned about:
+   - Forgetting to call ww_acquire_fini or ww_acquire_init.
+   - Attempting to lock more mutexes after ww_acquire_done.
+   - Attempting to lock the wrong mutex after -EDEADLK and
+     unlocking all mutexes.
+   - Attempting to lock the right mutex after -EDEADLK,
+     before unlocking all mutexes.
+
+   - Calling ww_mutex_lock_slow before -EDEADLK was returned.
+
+   - Unlocking mutexes with the wrong unlock function.
+   - Calling one of the ww_acquire_* twice on the same context.
+   - Using a different ww_class for the mutex than for the ww_acquire_ctx.
+   - Normal lockdep errors that can result in deadlocks.
+
+  Some of the lockdep errors that can result in deadlocks:
+   - Calling ww_acquire_init to initialize a second ww_acquire_ctx before
+     having called ww_acquire_fini on the first.
+   - 'normal' deadlocks that can occur.
+
+FIXME: Update this section once we have the TASK_DEADLOCK task state flag magic
+implemented.
diff --git a/include/linux/mutex-debug.h b/include/linux/mutex-debug.h
index 731d77d6e155..4ac8b1977b73 100644
--- a/include/linux/mutex-debug.h
+++ b/include/linux/mutex-debug.h
@@ -3,6 +3,7 @@
 
 #include <linux/linkage.h>
 #include <linux/lockdep.h>
+#include <linux/debug_locks.h>
 
 /*
  * Mutexes - debugging helpers:
diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index 433da8a1a426..a56b0ccc8a6c 100644
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -10,6 +10,7 @@
 #ifndef __LINUX_MUTEX_H
 #define __LINUX_MUTEX_H
 
+#include <asm/current.h>
 #include <linux/list.h>
 #include <linux/spinlock_types.h>
 #include <linux/linkage.h>
@@ -77,6 +78,36 @@ struct mutex_waiter {
 #endif
 };
 
+struct ww_class {
+        atomic_long_t stamp;
+        struct lock_class_key acquire_key;
+        struct lock_class_key mutex_key;
+        const char *acquire_name;
+        const char *mutex_name;
+};
+
+struct ww_acquire_ctx {
+        struct task_struct *task;
+        unsigned long stamp;
+        unsigned acquired;
+#ifdef CONFIG_DEBUG_MUTEXES
+        unsigned done_acquire;
+        struct ww_class *ww_class;
+        struct ww_mutex *contending_lock;
+#endif
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+        struct lockdep_map dep_map;
+#endif
+};
+
+struct ww_mutex {
+        struct mutex base;
+        struct ww_acquire_ctx *ctx;
+#ifdef CONFIG_DEBUG_MUTEXES
+        struct ww_class *ww_class;
+#endif
+};
+
 #ifdef CONFIG_DEBUG_MUTEXES
 # include <linux/mutex-debug.h>
 #else
@@ -101,8 +132,11 @@ static inline void mutex_destroy(struct mutex *lock) {}
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 # define __DEP_MAP_MUTEX_INITIALIZER(lockname) \
                 , .dep_map = { .name = #lockname }
+# define __WW_CLASS_MUTEX_INITIALIZER(lockname, ww_class) \
+                , .ww_class = &ww_class
 #else
 # define __DEP_MAP_MUTEX_INITIALIZER(lockname)
+# define __WW_CLASS_MUTEX_INITIALIZER(lockname, ww_class)
 #endif
 
 #define __MUTEX_INITIALIZER(lockname) \
@@ -112,13 +146,49 @@ static inline void mutex_destroy(struct mutex *lock) {}
                 __DEBUG_MUTEX_INITIALIZER(lockname) \
                 __DEP_MAP_MUTEX_INITIALIZER(lockname) }
 
+#define __WW_CLASS_INITIALIZER(ww_class) \
+                { .stamp = ATOMIC_LONG_INIT(0) \
+                , .acquire_name = #ww_class "_acquire" \
+                , .mutex_name = #ww_class "_mutex" }
+
+#define __WW_MUTEX_INITIALIZER(lockname, class) \
+                { .base = { \__MUTEX_INITIALIZER(lockname) } \
+                __WW_CLASS_MUTEX_INITIALIZER(lockname, class) }
+
 #define DEFINE_MUTEX(mutexname) \
         struct mutex mutexname = __MUTEX_INITIALIZER(mutexname)
 
+#define DEFINE_WW_CLASS(classname) \
+        struct ww_class classname = __WW_CLASS_INITIALIZER(classname)
+
+#define DEFINE_WW_MUTEX(mutexname, ww_class) \
+        struct ww_mutex mutexname = __WW_MUTEX_INITIALIZER(mutexname, ww_class)
+
+
 extern void __mutex_init(struct mutex *lock, const char *name,
                          struct lock_class_key *key);
 
 /**
+ * ww_mutex_init - initialize the w/w mutex
+ * @lock: the mutex to be initialized
+ * @ww_class: the w/w class the mutex should belong to
+ *
+ * Initialize the w/w mutex to unlocked state and associate it with the given
+ * class.
+ *
+ * It is not allowed to initialize an already locked mutex.
+ */
+static inline void ww_mutex_init(struct ww_mutex *lock,
+                                 struct ww_class *ww_class)
+{
+        __mutex_init(&lock->base, ww_class->mutex_name, &ww_class->mutex_key);
+        lock->ctx = NULL;
+#ifdef CONFIG_DEBUG_MUTEXES
+        lock->ww_class = ww_class;
+#endif
+}
+
+/**
  * mutex_is_locked - is the mutex locked
  * @lock: the mutex to be queried
  *
@@ -136,6 +206,7 @@ static inline int mutex_is_locked(struct mutex *lock)
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 extern void mutex_lock_nested(struct mutex *lock, unsigned int subclass);
 extern void _mutex_lock_nest_lock(struct mutex *lock, struct lockdep_map *nest_lock);
+
 extern int __must_check mutex_lock_interruptible_nested(struct mutex *lock,
                                         unsigned int subclass);
 extern int __must_check mutex_lock_killable_nested(struct mutex *lock,
@@ -147,7 +218,7 @@ extern int __must_check mutex_lock_killable_nested(struct mutex *lock,
 
 #define mutex_lock_nest_lock(lock, nest_lock)                           \
 do {                                                                    \
-        typecheck(struct lockdep_map *, &(nest_lock)->dep_map);         \
+        typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \
         _mutex_lock_nest_lock(lock, &(nest_lock)->dep_map);             \
 } while (0)
 
@@ -170,6 +241,288 @@ extern int __must_check mutex_lock_killable(struct mutex *lock);
  */
 extern int mutex_trylock(struct mutex *lock);
 extern void mutex_unlock(struct mutex *lock);
+
+/**
+ * ww_acquire_init - initialize a w/w acquire context
+ * @ctx: w/w acquire context to initialize
+ * @ww_class: w/w class of the context
+ *
+ * Initializes an context to acquire multiple mutexes of the given w/w class.
+ *
+ * Context-based w/w mutex acquiring can be done in any order whatsoever within
+ * a given lock class. Deadlocks will be detected and handled with the
+ * wait/wound logic.
+ *
+ * Mixing of context-based w/w mutex acquiring and single w/w mutex locking can
+ * result in undetected deadlocks and is so forbidden. Mixing different contexts
+ * for the same w/w class when acquiring mutexes can also result in undetected
+ * deadlocks, and is hence also forbidden. Both types of abuse will be caught by
+ * enabling CONFIG_PROVE_LOCKING.
+ *
+ * Nesting of acquire contexts for _different_ w/w classes is possible, subject
+ * to the usual locking rules between different lock classes.
+ *
+ * An acquire context must be released with ww_acquire_fini by the same task
+ * before the memory is freed. It is recommended to allocate the context itself
+ * on the stack.
+ */
+static inline void ww_acquire_init(struct ww_acquire_ctx *ctx,
+                                   struct ww_class *ww_class)
+{
+        ctx->task = current;
+        ctx->stamp = atomic_long_inc_return(&ww_class->stamp);
+        ctx->acquired = 0;
+#ifdef CONFIG_DEBUG_MUTEXES
+        ctx->ww_class = ww_class;
+        ctx->done_acquire = 0;
+        ctx->contending_lock = NULL;
+#endif
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+        debug_check_no_locks_freed((void *)ctx, sizeof(*ctx));
+        lockdep_init_map(&ctx->dep_map, ww_class->acquire_name,
+                         &ww_class->acquire_key, 0);
+        mutex_acquire(&ctx->dep_map, 0, 0, _RET_IP_);
+#endif
+}
+
+/**
+ * ww_acquire_done - marks the end of the acquire phase
+ * @ctx: the acquire context
+ *
+ * Marks the end of the acquire phase, any further w/w mutex lock calls using
+ * this context are forbidden.
+ *
+ * Calling this function is optional, it is just useful to document w/w mutex
+ * code and clearly designated the acquire phase from actually using the locked
+ * data structures.
+ */
+static inline void ww_acquire_done(struct ww_acquire_ctx *ctx)
+{
+#ifdef CONFIG_DEBUG_MUTEXES
+        lockdep_assert_held(ctx);
+
+        DEBUG_LOCKS_WARN_ON(ctx->done_acquire);
+        ctx->done_acquire = 1;
+#endif
+}
+
+/**
+ * ww_acquire_fini - releases a w/w acquire context
+ * @ctx: the acquire context to free
+ *
+ * Releases a w/w acquire context. This must be called _after_ all acquired w/w
+ * mutexes have been released with ww_mutex_unlock.
+ */
+static inline void ww_acquire_fini(struct ww_acquire_ctx *ctx)
+{
+#ifdef CONFIG_DEBUG_MUTEXES
+        mutex_release(&ctx->dep_map, 0, _THIS_IP_);
+
+        DEBUG_LOCKS_WARN_ON(ctx->acquired);
+        if (!config_enabled(CONFIG_PROVE_LOCKING))
+                /*
+                 * lockdep will normally handle this,
+                 * but fail without anyway
+                 */
+                ctx->done_acquire = 1;
+
+        if (!config_enabled(CONFIG_DEBUG_LOCK_ALLOC))
+                /* ensure ww_acquire_fini will still fail if called twice */
+                ctx->acquired = ~0U;
+#endif
+}
+
+extern int __must_check __ww_mutex_lock(struct ww_mutex *lock,
+                                        struct ww_acquire_ctx *ctx);
+extern int __must_check __ww_mutex_lock_interruptible(struct ww_mutex *lock,
+                                                      struct ww_acquire_ctx *ctx);
+
+/**
+ * ww_mutex_lock - acquire the w/w mutex
+ * @lock: the mutex to be acquired
+ * @ctx: w/w acquire context, or NULL to acquire only a single lock.
+ *
+ * Lock the w/w mutex exclusively for this task.
+ *
+ * Deadlocks within a given w/w class of locks are detected and handled with the
+ * wait/wound algorithm. If the lock isn't immediately avaiable this function
+ * will either sleep until it is (wait case). Or it selects the current context
+ * for backing off by returning -EDEADLK (wound case). Trying to acquire the
+ * same lock with the same context twice is also detected and signalled by
+ * returning -EALREADY. Returns 0 if the mutex was successfully acquired.
+ *
+ * In the wound case the caller must release all currently held w/w mutexes for
+ * the given context and then wait for this contending lock to be available by
+ * calling ww_mutex_lock_slow. Alternatively callers can opt to not acquire this
+ * lock and proceed with trying to acquire further w/w mutexes (e.g. when
+ * scanning through lru lists trying to free resources).
+ *
+ * The mutex must later on be released by the same task that
+ * acquired it. The task may not exit without first unlocking the mutex. Also,
+ * kernel memory where the mutex resides must not be freed with the mutex still
+ * locked. The mutex must first be initialized (or statically defined) before it
+ * can be locked. memset()-ing the mutex to 0 is not allowed. The mutex must be
+ * of the same w/w lock class as was used to initialize the acquire context.
+ *
+ * A mutex acquired with this function must be released with ww_mutex_unlock.
+ */
+static inline int ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
+{
+        if (ctx)
+                return __ww_mutex_lock(lock, ctx);
+        else {
+                mutex_lock(&lock->base);
+                return 0;
+        }
+}
+
+/**
+ * ww_mutex_lock_interruptible - acquire the w/w mutex, interruptible
+ * @lock: the mutex to be acquired
+ * @ctx: w/w acquire context
+ *
+ * Lock the w/w mutex exclusively for this task.
+ *
+ * Deadlocks within a given w/w class of locks are detected and handled with the
+ * wait/wound algorithm. If the lock isn't immediately avaiable this function
+ * will either sleep until it is (wait case). Or it selects the current context
+ * for backing off by returning -EDEADLK (wound case). Trying to acquire the
+ * same lock with the same context twice is also detected and signalled by
+ * returning -EALREADY. Returns 0 if the mutex was successfully acquired. If a
+ * signal arrives while waiting for the lock then this function returns -EINTR.
+ *
+ * In the wound case the caller must release all currently held w/w mutexes for
+ * the given context and then wait for this contending lock to be available by
+ * calling ww_mutex_lock_slow_interruptible. Alternatively callers can opt to
+ * not acquire this lock and proceed with trying to acquire further w/w mutexes
+ * (e.g. when scanning through lru lists trying to free resources).
+ *
+ * The mutex must later on be released by the same task that
+ * acquired it. The task may not exit without first unlocking the mutex. Also,
+ * kernel memory where the mutex resides must not be freed with the mutex still
+ * locked. The mutex must first be initialized (or statically defined) before it
+ * can be locked. memset()-ing the mutex to 0 is not allowed. The mutex must be
+ * of the same w/w lock class as was used to initialize the acquire context.
+ *
+ * A mutex acquired with this function must be released with ww_mutex_unlock.
+ */
+static inline int __must_check ww_mutex_lock_interruptible(struct ww_mutex *lock,
+                                                           struct ww_acquire_ctx *ctx)
+{
+        if (ctx)
+                return __ww_mutex_lock_interruptible(lock, ctx);
+        else
+                return mutex_lock_interruptible(&lock->base);
+}
+
+/**
+ * ww_mutex_lock_slow - slowpath acquiring of the w/w mutex
+ * @lock: the mutex to be acquired
+ * @ctx: w/w acquire context
+ *
+ * Acquires a w/w mutex with the given context after a wound case. This function
+ * will sleep until the lock becomes available.
+ *
+ * The caller must have released all w/w mutexes already acquired with the
+ * context and then call this function on the contended lock.
+ *
+ * Afterwards the caller may continue to (re)acquire the other w/w mutexes it
+ * needs with ww_mutex_lock. Note that the -EALREADY return code from
+ * ww_mutex_lock can be used to avoid locking this contended mutex twice.
+ *
+ * It is forbidden to call this function with any other w/w mutexes associated
+ * with the context held. It is forbidden to call this on anything else than the
+ * contending mutex.
+ *
+ * Note that the slowpath lock acquiring can also be done by calling
+ * ww_mutex_lock directly. This function here is simply to help w/w mutex
+ * locking code readability by clearly denoting the slowpath.
+ */
+static inline void
+ww_mutex_lock_slow(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
+{
+        int ret;
+#ifdef CONFIG_DEBUG_MUTEXES
+        DEBUG_LOCKS_WARN_ON(!ctx->contending_lock);
+#endif
+        ret = ww_mutex_lock(lock, ctx);
+        (void)ret;
+}
+
+/**
+ * ww_mutex_lock_slow_interruptible - slowpath acquiring of the w/w mutex,
+ *                                    interruptible
+ * @lock: the mutex to be acquired
+ * @ctx: w/w acquire context
+ *
+ * Acquires a w/w mutex with the given context after a wound case. This function
+ * will sleep until the lock becomes available and returns 0 when the lock has
+ * been acquired. If a signal arrives while waiting for the lock then this
+ * function returns -EINTR.
+ *
+ * The caller must have released all w/w mutexes already acquired with the
+ * context and then call this function on the contended lock.
+ *
+ * Afterwards the caller may continue to (re)acquire the other w/w mutexes it
+ * needs with ww_mutex_lock. Note that the -EALREADY return code from
+ * ww_mutex_lock can be used to avoid locking this contended mutex twice.
+ *
+ * It is forbidden to call this function with any other w/w mutexes associated
+ * with the given context held. It is forbidden to call this on anything else
+ * than the contending mutex.
+ *
+ * Note that the slowpath lock acquiring can also be done by calling
+ * ww_mutex_lock_interruptible directly. This function here is simply to help
+ * w/w mutex locking code readability by clearly denoting the slowpath.
+ */
+static inline int __must_check
+ww_mutex_lock_slow_interruptible(struct ww_mutex *lock,
+                                 struct ww_acquire_ctx *ctx)
+{
+#ifdef CONFIG_DEBUG_MUTEXES
+        DEBUG_LOCKS_WARN_ON(!ctx->contending_lock);
+#endif
+        return ww_mutex_lock_interruptible(lock, ctx);
+}
+
+extern void ww_mutex_unlock(struct ww_mutex *lock);
+
+/**
+ * ww_mutex_trylock - tries to acquire the w/w mutex without acquire context
+ * @lock: mutex to lock
+ *
+ * Trylocks a mutex without acquire context, so no deadlock detection is
+ * possible. Returns 1 if the mutex has been acquired successfully, 0 otherwise.
+ */
+static inline int __must_check ww_mutex_trylock(struct ww_mutex *lock)
+{
+        return mutex_trylock(&lock->base);
+}
+
+/***
+ * ww_mutex_destroy - mark a w/w mutex unusable
+ * @lock: the mutex to be destroyed
+ *
+ * This function marks the mutex uninitialized, and any subsequent
+ * use of the mutex is forbidden. The mutex must not be locked when
+ * this function is called.
+ */
+static inline void ww_mutex_destroy(struct ww_mutex *lock)
+{
+        mutex_destroy(&lock->base);
+}
+
+/**
+ * ww_mutex_is_locked - is the w/w mutex locked
+ * @lock: the mutex to be queried
+ *
+ * Returns 1 if the mutex is locked, 0 if unlocked.
+ */
+static inline bool ww_mutex_is_locked(struct ww_mutex *lock)
+{
+        return mutex_is_locked(&lock->base);
+}
+
 extern int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);
 
 #ifndef CONFIG_HAVE_ARCH_MUTEX_CPU_RELAX
diff --git a/kernel/mutex.c b/kernel/mutex.c
index 42f8dda2467b..fc801aafe8fd 100644
--- a/kernel/mutex.c
+++ b/kernel/mutex.c
@@ -254,16 +254,165 @@ void __sched mutex_unlock(struct mutex *lock)
 
 EXPORT_SYMBOL(mutex_unlock);
 
+/**
+ * ww_mutex_unlock - release the w/w mutex
+ * @lock: the mutex to be released
+ *
+ * Unlock a mutex that has been locked by this task previously with any of the
+ * ww_mutex_lock* functions (with or without an acquire context). It is
+ * forbidden to release the locks after releasing the acquire context.
+ *
+ * This function must not be used in interrupt context. Unlocking
+ * of a unlocked mutex is not allowed.
+ */
+void __sched ww_mutex_unlock(struct ww_mutex *lock)
+{
+        /*
+         * The unlocking fastpath is the 0->1 transition from 'locked'
+         * into 'unlocked' state:
+         */
+        if (lock->ctx) {
+#ifdef CONFIG_DEBUG_MUTEXES
+                DEBUG_LOCKS_WARN_ON(!lock->ctx->acquired);
+#endif
+                if (lock->ctx->acquired > 0)
+                        lock->ctx->acquired--;
+                lock->ctx = NULL;
+        }
+
+#ifndef CONFIG_DEBUG_MUTEXES
+        /*
+         * When debugging is enabled we must not clear the owner before time,
+         * the slow path will always be taken, and that clears the owner field
+         * after verifying that it was indeed current.
+         */
+        mutex_clear_owner(&lock->base);
+#endif
+        __mutex_fastpath_unlock(&lock->base.count, __mutex_unlock_slowpath);
+}
+EXPORT_SYMBOL(ww_mutex_unlock);
+
+static inline int __sched
+__mutex_lock_check_stamp(struct mutex *lock, struct ww_acquire_ctx *ctx)
+{
+        struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
+        struct ww_acquire_ctx *hold_ctx = ACCESS_ONCE(ww->ctx);
+
+        if (!hold_ctx)
+                return 0;
+
+        if (unlikely(ctx == hold_ctx))
+                return -EALREADY;
+
+        if (ctx->stamp - hold_ctx->stamp <= LONG_MAX &&
+            (ctx->stamp != hold_ctx->stamp || ctx > hold_ctx)) {
+#ifdef CONFIG_DEBUG_MUTEXES
+                DEBUG_LOCKS_WARN_ON(ctx->contending_lock);
+                ctx->contending_lock = ww;
+#endif
+                return -EDEADLK;
+        }
+
+        return 0;
+}
+
+static __always_inline void ww_mutex_lock_acquired(struct ww_mutex *ww,
+                                                   struct ww_acquire_ctx *ww_ctx)
+{
+#ifdef CONFIG_DEBUG_MUTEXES
+        /*
+         * If this WARN_ON triggers, you used ww_mutex_lock to acquire,
+         * but released with a normal mutex_unlock in this call.
+         *
+         * This should never happen, always use ww_mutex_unlock.
+         */
+        DEBUG_LOCKS_WARN_ON(ww->ctx);
+
+        /*
+         * Not quite done after calling ww_acquire_done() ?
+         */
+        DEBUG_LOCKS_WARN_ON(ww_ctx->done_acquire);
+
+        if (ww_ctx->contending_lock) {
+                /*
+                 * After -EDEADLK you tried to
+                 * acquire a different ww_mutex? Bad!
+                 */
+                DEBUG_LOCKS_WARN_ON(ww_ctx->contending_lock != ww);
+
+                /*
+                 * You called ww_mutex_lock after receiving -EDEADLK,
+                 * but 'forgot' to unlock everything else first?
+                 */
+                DEBUG_LOCKS_WARN_ON(ww_ctx->acquired > 0);
+                ww_ctx->contending_lock = NULL;
+        }
+
+        /*
+         * Naughty, using a different class will lead to undefined behavior!
+         */
+        DEBUG_LOCKS_WARN_ON(ww_ctx->ww_class != ww->ww_class);
+#endif
+        ww_ctx->acquired++;
+}
+
+/*
+ * after acquiring lock with fastpath or when we lost out in contested
+ * slowpath, set ctx and wake up any waiters so they can recheck.
+ *
+ * This function is never called when CONFIG_DEBUG_LOCK_ALLOC is set,
+ * as the fastpath and opportunistic spinning are disabled in that case.
+ */
+static __always_inline void
+ww_mutex_set_context_fastpath(struct ww_mutex *lock,
+                               struct ww_acquire_ctx *ctx)
+{
+        unsigned long flags;
+        struct mutex_waiter *cur;
+
+        ww_mutex_lock_acquired(lock, ctx);
+
+        lock->ctx = ctx;
+
+        /*
+         * The lock->ctx update should be visible on all cores before
+         * the atomic read is done, otherwise contended waiters might be
+         * missed. The contended waiters will either see ww_ctx == NULL
+         * and keep spinning, or it will acquire wait_lock, add itself
+         * to waiter list and sleep.
+         */
+        smp_mb(); /* ^^^ */
+
+        /*
+         * Check if lock is contended, if not there is nobody to wake up
+         */
+        if (likely(atomic_read(&lock->base.count) == 0))
+                return;
+
+        /*
+         * Uh oh, we raced in fastpath, wake up everyone in this case,
+         * so they can see the new lock->ctx.
+         */
+        spin_lock_mutex(&lock->base.wait_lock, flags);
+        list_for_each_entry(cur, &lock->base.wait_list, list) {
+                debug_mutex_wake_waiter(&lock->base, cur);
+                wake_up_process(cur->task);
+        }
+        spin_unlock_mutex(&lock->base.wait_lock, flags);
+}
+
 /*
  * Lock a mutex (possibly interruptible), slowpath:
  */
-static inline int __sched
+static __always_inline int __sched
 __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
-                    struct lockdep_map *nest_lock, unsigned long ip)
+                    struct lockdep_map *nest_lock, unsigned long ip,
+                    struct ww_acquire_ctx *ww_ctx)
 {
         struct task_struct *task = current;
         struct mutex_waiter waiter;
         unsigned long flags;
+        int ret;
 
         preempt_disable();
         mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);
@@ -298,6 +447,22 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
                 struct task_struct *owner;
                 struct mspin_node  node;
 
+                if (!__builtin_constant_p(ww_ctx == NULL) && ww_ctx->acquired > 0) {
+                        struct ww_mutex *ww;
+
+                        ww = container_of(lock, struct ww_mutex, base);
+                        /*
+                         * If ww->ctx is set the contents are undefined, only
+                         * by acquiring wait_lock there is a guarantee that
+                         * they are not invalid when reading.
+                         *
+                         * As such, when deadlock detection needs to be
+                         * performed the optimistic spinning cannot be done.
+                         */
+                        if (ACCESS_ONCE(ww->ctx))
+                                break;
+                }
+
                 /*
                  * If there's an owner, wait for it to either
                  * release the lock or go to sleep.
@@ -312,6 +477,13 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
                 if ((atomic_read(&lock->count) == 1) &&
                     (atomic_cmpxchg(&lock->count, 1, 0) == 1)) {
                         lock_acquired(&lock->dep_map, ip);
+                        if (!__builtin_constant_p(ww_ctx == NULL)) {
+                                struct ww_mutex *ww;
+                                ww = container_of(lock, struct ww_mutex, base);
+
+                                ww_mutex_set_context_fastpath(ww, ww_ctx);
+                        }
+
                         mutex_set_owner(lock);
                         mspin_unlock(MLOCK(lock), &node);
                         preempt_enable();
@@ -371,15 +543,16 @@ slowpath:
                  * TASK_UNINTERRUPTIBLE case.)
                  */
                 if (unlikely(signal_pending_state(state, task))) {
-                        mutex_remove_waiter(lock, &waiter,
-                                            task_thread_info(task));
-                        mutex_release(&lock->dep_map, 1, ip);
-                        spin_unlock_mutex(&lock->wait_lock, flags);
+                        ret = -EINTR;
+                        goto err;
+                }
 
-                        debug_mutex_free_waiter(&waiter);
-                        preempt_enable();
-                        return -EINTR;
+                if (!__builtin_constant_p(ww_ctx == NULL) && ww_ctx->acquired > 0) {
+                        ret = __mutex_lock_check_stamp(lock, ww_ctx);
+                        if (ret)
+                                goto err;
                 }
+
                 __set_task_state(task, state);
 
                 /* didn't get the lock, go to sleep: */
@@ -394,6 +567,30 @@ done:
         mutex_remove_waiter(lock, &waiter, current_thread_info());
         mutex_set_owner(lock);
 
+        if (!__builtin_constant_p(ww_ctx == NULL)) {
+                struct ww_mutex *ww = container_of(lock,
+                                                      struct ww_mutex,
+                                                      base);
+                struct mutex_waiter *cur;
+
+                /*
+                 * This branch gets optimized out for the common case,
+                 * and is only important for ww_mutex_lock.
+                 */
+
+                ww_mutex_lock_acquired(ww, ww_ctx);
+                ww->ctx = ww_ctx;
+
+                /*
+                 * Give any possible sleeping processes the chance to wake up,
+                 * so they can recheck if they have to back off.
+                 */
+                list_for_each_entry(cur, &lock->wait_list, list) {
+                        debug_mutex_wake_waiter(lock, cur);
+                        wake_up_process(cur->task);
+                }
+        }
+
         /* set it to 0 if there are no waiters left: */
         if (likely(list_empty(&lock->wait_list)))
                 atomic_set(&lock->count, 0);
@@ -404,6 +601,14 @@ done:
         preempt_enable();
 
         return 0;
+
+err:
+        mutex_remove_waiter(lock, &waiter, task_thread_info(task));
+        spin_unlock_mutex(&lock->wait_lock, flags);
+        debug_mutex_free_waiter(&waiter);
+        mutex_release(&lock->dep_map, 1, ip);
+        preempt_enable();
+        return ret;
 }
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
@@ -411,7 +616,8 @@ void __sched
 mutex_lock_nested(struct mutex *lock, unsigned int subclass)
 {
         might_sleep();
-        __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, subclass, NULL, _RET_IP_);
+        __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE,
+                            subclass, NULL, _RET_IP_, NULL);
 }
 
 EXPORT_SYMBOL_GPL(mutex_lock_nested);
@@ -420,7 +626,8 @@ void __sched
 _mutex_lock_nest_lock(struct mutex *lock, struct lockdep_map *nest)
 {
         might_sleep();
-        __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, nest, _RET_IP_);
+        __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE,
+                            0, nest, _RET_IP_, NULL);
 }
 
 EXPORT_SYMBOL_GPL(_mutex_lock_nest_lock);
@@ -429,7 +636,8 @@ int __sched
 mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass)
 {
         might_sleep();
-        return __mutex_lock_common(lock, TASK_KILLABLE, subclass, NULL, _RET_IP_);
+        return __mutex_lock_common(lock, TASK_KILLABLE,
+                                   subclass, NULL, _RET_IP_, NULL);
 }
 EXPORT_SYMBOL_GPL(mutex_lock_killable_nested);
 
@@ -438,10 +646,30 @@ mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass)
 {
         might_sleep();
         return __mutex_lock_common(lock, TASK_INTERRUPTIBLE,
-                                   subclass, NULL, _RET_IP_);
+                                   subclass, NULL, _RET_IP_, NULL);
 }
 
 EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested);
+
+
+int __sched
+__ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
+{
+        might_sleep();
+        return __mutex_lock_common(&lock->base, TASK_UNINTERRUPTIBLE,
+                                   0, &ctx->dep_map, _RET_IP_, ctx);
+}
+EXPORT_SYMBOL_GPL(__ww_mutex_lock);
+
+int __sched
+__ww_mutex_lock_interruptible(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
+{
+        might_sleep();
+        return __mutex_lock_common(&lock->base, TASK_INTERRUPTIBLE,
+                                   0, &ctx->dep_map, _RET_IP_, ctx);
+}
+EXPORT_SYMBOL_GPL(__ww_mutex_lock_interruptible);
+
 #endif
 
 /*
@@ -544,20 +772,39 @@ __mutex_lock_slowpath(atomic_t *lock_count)
 {
         struct mutex *lock = container_of(lock_count, struct mutex, count);
 
-        __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, NULL, _RET_IP_);
+        __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0,
+                            NULL, _RET_IP_, NULL);
 }
 
 static noinline int __sched
 __mutex_lock_killable_slowpath(struct mutex *lock)
 {
-        return __mutex_lock_common(lock, TASK_KILLABLE, 0, NULL, _RET_IP_);
+        return __mutex_lock_common(lock, TASK_KILLABLE, 0,
+                                   NULL, _RET_IP_, NULL);
 }
 
 static noinline int __sched
 __mutex_lock_interruptible_slowpath(struct mutex *lock)
 {
-        return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 0, NULL, _RET_IP_);
+        return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 0,
+                                   NULL, _RET_IP_, NULL);
+}
+
+static noinline int __sched
+__ww_mutex_lock_slowpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
+{
+        return __mutex_lock_common(&lock->base, TASK_UNINTERRUPTIBLE, 0,
+                                   NULL, _RET_IP_, ctx);
 }
+
+static noinline int __sched
+__ww_mutex_lock_interruptible_slowpath(struct ww_mutex *lock,
+                                            struct ww_acquire_ctx *ctx)
+{
+        return __mutex_lock_common(&lock->base, TASK_INTERRUPTIBLE, 0,
+                                   NULL, _RET_IP_, ctx);
+}
+
 #endif
 
 /*
@@ -613,6 +860,45 @@ int __sched mutex_trylock(struct mutex *lock)
 }
 EXPORT_SYMBOL(mutex_trylock);
 
+#ifndef CONFIG_DEBUG_LOCK_ALLOC
+int __sched
+__ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
+{
+        int ret;
+
+        might_sleep();
+
+        ret = __mutex_fastpath_lock_retval(&lock->base.count);
+
+        if (likely(!ret)) {
+                ww_mutex_set_context_fastpath(lock, ctx);
+                mutex_set_owner(&lock->base);
+        } else
+                ret = __ww_mutex_lock_slowpath(lock, ctx);
+        return ret;
+}
+EXPORT_SYMBOL(__ww_mutex_lock);
+
+int __sched
+__ww_mutex_lock_interruptible(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
+{
+        int ret;
+
+        might_sleep();
+
+        ret = __mutex_fastpath_lock_retval(&lock->base.count);
+
+        if (likely(!ret)) {
+                ww_mutex_set_context_fastpath(lock, ctx);
+                mutex_set_owner(&lock->base);
+        } else
+                ret = __ww_mutex_lock_interruptible_slowpath(lock, ctx);
+        return ret;
+}
+EXPORT_SYMBOL(__ww_mutex_lock_interruptible);
+
+#endif
+
 /**
  * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0
  * @cnt: the atomic which we are to dec
diff --git a/lib/debug_locks.c b/lib/debug_locks.c
index f2fa60c59343..96c4c633d95e 100644
--- a/lib/debug_locks.c
+++ b/lib/debug_locks.c
@@ -30,6 +30,7 @@ EXPORT_SYMBOL_GPL(debug_locks);
  * a locking bug is detected.
  */
 int debug_locks_silent;
+EXPORT_SYMBOL_GPL(debug_locks_silent);
 
 /*
  * Generic 'turn off all lock debugging' function:
@@ -44,3 +45,4 @@ int debug_locks_off(void)
         }
         return 0;
 }
+EXPORT_SYMBOL_GPL(debug_locks_off);