|author||Tony Breeds <firstname.lastname@example.org>||2010-05-19 15:46:36 +1000|
|committer||Ingo Molnar <email@example.com>||2010-05-19 08:18:44 +0200|
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 <firstname.lastname@example.org> Acked-by: Linus Torvalds <email@example.com> Acked-by: Peter Zijlstra <firstname.lastname@example.org> Cc: Benjamin Herrenschmidt <email@example.com> Cc: <firstname.lastname@example.org> LKML-Reference: <20100519054636.GC12389@ozlabs.org> [ added an unlikely() attribute to the branch ] Signed-off-by: Ingo Molnar <email@example.com>
1 files changed, 7 insertions, 0 deletions
diff --git a/kernel/mutex.c b/kernel/mutex.c
index 632f04c57d8..4c0b7b3e6d2 100644
@@ -172,6 +172,13 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
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))
* If there's an owner, wait for it to either
* release the lock or go to sleep.