3 files changed, 62 insertions, 3 deletions

diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index a56b0ccc8a6c..3793ed7feeeb 100644
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -98,6 +98,10 @@ struct ww_acquire_ctx {
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
         struct lockdep_map dep_map;
 #endif
+#ifdef CONFIG_DEBUG_WW_MUTEX_SLOWPATH
+        unsigned deadlock_inject_interval;
+        unsigned deadlock_inject_countdown;
+#endif
 };
 
 struct ww_mutex {
@@ -283,6 +287,10 @@ static inline void ww_acquire_init(struct ww_acquire_ctx *ctx,
                          &ww_class->acquire_key, 0);
         mutex_acquire(&ctx->dep_map, 0, 0, _RET_IP_);
 #endif
+#ifdef CONFIG_DEBUG_WW_MUTEX_SLOWPATH
+        ctx->deadlock_inject_interval = 1;
+        ctx->deadlock_inject_countdown = ctx->stamp & 0xf;
+#endif
 }
 
 /**
diff --git a/kernel/mutex.c b/kernel/mutex.c
index fc801aafe8fd..e581ada5faf4 100644
--- a/kernel/mutex.c
+++ b/kernel/mutex.c
@@ -651,22 +651,60 @@ mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass)
 
 EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested);
 
+static inline int
+ww_mutex_deadlock_injection(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
+{
+#ifdef CONFIG_DEBUG_WW_MUTEX_SLOWPATH
+        unsigned tmp;
+
+        if (ctx->deadlock_inject_countdown-- == 0) {
+                tmp = ctx->deadlock_inject_interval;
+                if (tmp > UINT_MAX/4)
+                        tmp = UINT_MAX;
+                else
+                        tmp = tmp*2 + tmp + tmp/2;
+
+                ctx->deadlock_inject_interval = tmp;
+                ctx->deadlock_inject_countdown = tmp;
+                ctx->contending_lock = lock;
+
+                ww_mutex_unlock(lock);
+
+                return -EDEADLK;
+        }
+#endif
+
+        return 0;
+}
 
 int __sched
 __ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
 {
+        int ret;
+
         might_sleep();
-        return __mutex_lock_common(&lock->base, TASK_UNINTERRUPTIBLE,
+        ret =  __mutex_lock_common(&lock->base, TASK_UNINTERRUPTIBLE,
                                    0, &ctx->dep_map, _RET_IP_, ctx);
+        if (!ret && ctx->acquired > 0)
+                return ww_mutex_deadlock_injection(lock, ctx);
+
+        return ret;
 }
 EXPORT_SYMBOL_GPL(__ww_mutex_lock);
 
 int __sched
 __ww_mutex_lock_interruptible(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
 {
+        int ret;
+
         might_sleep();
-        return __mutex_lock_common(&lock->base, TASK_INTERRUPTIBLE,
-                                   0, &ctx->dep_map, _RET_IP_, ctx);
+        ret = __mutex_lock_common(&lock->base, TASK_INTERRUPTIBLE,
+                                  0, &ctx->dep_map, _RET_IP_, ctx);
+
+        if (!ret && ctx->acquired > 0)
+                return ww_mutex_deadlock_injection(lock, ctx);
+
+        return ret;
 }
 EXPORT_SYMBOL_GPL(__ww_mutex_lock_interruptible);
 
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 566cf2bc08ea..7154f799541a 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -547,6 +547,19 @@ config DEBUG_MUTEXES
          This feature allows mutex semantics violations to be detected and
          reported.
 
+config DEBUG_WW_MUTEX_SLOWPATH
+        bool "Wait/wound mutex debugging: Slowpath testing"
+        depends on DEBUG_KERNEL && TRACE_IRQFLAGS_SUPPORT && STACKTRACE_SUPPORT && LOCKDEP_SUPPORT
+        select DEBUG_LOCK_ALLOC
+        select DEBUG_SPINLOCK
+        select DEBUG_MUTEXES
+        help
+         This feature enables slowpath testing for w/w mutex users by
+         injecting additional -EDEADLK wound/backoff cases. Together with
+         the full mutex checks enabled with (CONFIG_PROVE_LOCKING) this
+         will test all possible w/w mutex interface abuse with the
+         exception of simply not acquiring all the required locks.
+
 config DEBUG_LOCK_ALLOC
         bool "Lock debugging: detect incorrect freeing of live locks"
         depends on DEBUG_KERNEL && TRACE_IRQFLAGS_SUPPORT && STACKTRACE_SUPPORT && LOCKDEP_SUPPORT