lockdep: Implement lock pinning

Add a lockdep annotation that WARNs if you 'accidentially' unlock a
lock.

This is especially helpful for code with callbacks, where the upper
layer assumes a lock remains taken but a lower layer thinks it maybe
can drop and reacquire the lock.

By unwittingly breaking up the lock, races can be introduced.

Lock pinning is a lockdep annotation that helps with this, when you
lockdep_pin_lock() a held lock, any unlock without a
lockdep_unpin_lock() will produce a WARN. Think of this as a relative
of lockdep_assert_held(), except you don't only assert its held now,
but ensure it stays held until you release your assertion.

RFC: a possible alternative API would be something like:

  int cookie = lockdep_pin_lock(&foo);
  ...
  lockdep_unpin_lock(&foo, cookie);

Where we pick a random number for the pin_count; this makes it
impossible to sneak a lock break in without also passing the right
cookie along.

I've not done this because it ends up generating code for !LOCKDEP,
esp. if you need to pass the cookie around for some reason.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: ktkhai@parallels.com
Cc: rostedt@goodmis.org
Cc: juri.lelli@gmail.com
Cc: pang.xunlei@linaro.org
Cc: oleg@redhat.com
Cc: wanpeng.li@linux.intel.com
Cc: umgwanakikbuti@gmail.com
Link: http://lkml.kernel.org/r/20150611124743.906731065@infradead.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 80 insertions, 0 deletions

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index a266d5165b63..18f9f434d17e 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -3157,6 +3157,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
         hlock->waittime_stamp = 0;
         hlock->holdtime_stamp = lockstat_clock();
 #endif
+        hlock->pin_count = 0;
 
         if (check && !mark_irqflags(curr, hlock))
                 return 0;
@@ -3403,6 +3404,8 @@ found_it:
         if (hlock->instance == lock)
                 lock_release_holdtime(hlock);
 
+        WARN(hlock->pin_count, "releasing a pinned lock\n");
+
         if (hlock->references) {
                 hlock->references--;
                 if (hlock->references) {
@@ -3459,6 +3462,49 @@ static int __lock_is_held(struct lockdep_map *lock)
         return 0;
 }
 
+static void __lock_pin_lock(struct lockdep_map *lock)
+{
+        struct task_struct *curr = current;
+        int i;
+
+        if (unlikely(!debug_locks))
+                return;
+
+        for (i = 0; i < curr->lockdep_depth; i++) {
+                struct held_lock *hlock = curr->held_locks + i;
+
+                if (match_held_lock(hlock, lock)) {
+                        hlock->pin_count++;
+                        return;
+                }
+        }
+
+        WARN(1, "pinning an unheld lock\n");
+}
+
+static void __lock_unpin_lock(struct lockdep_map *lock)
+{
+        struct task_struct *curr = current;
+        int i;
+
+        if (unlikely(!debug_locks))
+                return;
+
+        for (i = 0; i < curr->lockdep_depth; i++) {
+                struct held_lock *hlock = curr->held_locks + i;
+
+                if (match_held_lock(hlock, lock)) {
+                        if (WARN(!hlock->pin_count, "unpinning an unpinned lock\n"))
+                                return;
+
+                        hlock->pin_count--;
+                        return;
+                }
+        }
+
+        WARN(1, "unpinning an unheld lock\n");
+}
+
 /*
  * Check whether we follow the irq-flags state precisely:
  */
@@ -3582,6 +3628,40 @@ int lock_is_held(struct lockdep_map *lock)
 }
 EXPORT_SYMBOL_GPL(lock_is_held);
 
+void lock_pin_lock(struct lockdep_map *lock)
+{
+        unsigned long flags;
+
+        if (unlikely(current->lockdep_recursion))
+                return;
+
+        raw_local_irq_save(flags);
+        check_flags(flags);
+
+        current->lockdep_recursion = 1;
+        __lock_pin_lock(lock);
+        current->lockdep_recursion = 0;
+        raw_local_irq_restore(flags);
+}
+EXPORT_SYMBOL_GPL(lock_pin_lock);
+
+void lock_unpin_lock(struct lockdep_map *lock)
+{
+        unsigned long flags;
+
+        if (unlikely(current->lockdep_recursion))
+                return;
+
+        raw_local_irq_save(flags);
+        check_flags(flags);
+
+        current->lockdep_recursion = 1;
+        __lock_unpin_lock(lock);
+        current->lockdep_recursion = 0;
+        raw_local_irq_restore(flags);
+}
+EXPORT_SYMBOL_GPL(lock_unpin_lock);
+
 void lockdep_set_current_reclaim_state(gfp_t gfp_mask)
 {
         current->lockdep_reclaim_gfp = gfp_mask;