mutex: Fix optimistic spinning vs. BKL

Currently, we can hit a nasty case with optimistic
spinning on mutexes:

    CPU A tries to take a mutex, while holding the BKL

    CPU B tried to take the BLK while holding the mutex

This looks like a AB-BA scenario but in practice, is
allowed and happens due to the auto-release on
schedule() nature of the BKL.

In that case, the optimistic spinning code can get us
into a situation where instead of going to sleep, A
will spin waiting for B who is spinning waiting for
A, and the only way out of that loop is the
need_resched() test in mutex_spin_on_owner().

This patch fixes it by completely disabling spinning
if we own the BKL. This adds one more detail to the
extensive list of reasons why it's a bad idea for
kernel code to be holding the BKL.

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: <stable@kernel.org>
LKML-Reference: <20100519054636.GC12389@ozlabs.org>
[ added an unlikely() attribute to the branch ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/mutex.c b/kernel/mutex.c
index 632f04c57d82b0207608f10a57f95582833489c1..4c0b7b3e6d2e9a483c6cb4cc384e979911ed03bb 100644 (file)
--- a/kernel/mutex.c
+++ b/kernel/mutex.c
@@ -171,6 +171,13 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
        for (;;) {
                struct thread_info *owner;
 
+               /*
+                * If we own the BKL, then don't spin. The owner of
+                * the mutex might be waiting on us to release the BKL.
+                */
+               if (unlikely(current->lock_depth >= 0))
+                       break;
+
                /*
                 * If there's an owner, wait for it to either
                 * release the lock or go to sleep.