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authorWill Deacon <will.deacon@arm.com>2012-08-10 14:22:09 (GMT)
committerThomas Gleixner <tglx@linutronix.de>2012-08-13 16:46:54 (GMT)
commit0bce9c46bf3b15f485d82d7e81dabed6ebcc24b1 (patch)
tree2378c052d6a19efaad21eabd58ccf8b8df489bde /include/asm-generic
parent3bf671af14d591ede9251acb0085e8017f3705e7 (diff)
downloadlinux-0bce9c46bf3b15f485d82d7e81dabed6ebcc24b1.tar.xz
mutex: Place lock in contended state after fastpath_lock failure
ARM recently moved to asm-generic/mutex-xchg.h for its mutex implementation after the previous implementation was found to be missing some crucial memory barriers. However, this has revealed some problems running hackbench on SMP platforms due to the way in which the MUTEX_SPIN_ON_OWNER code operates. The symptoms are that a bunch of hackbench tasks are left waiting on an unlocked mutex and therefore never get woken up to claim it. This boils down to the following sequence of events: Task A Task B Task C Lock value 0 1 1 lock() 0 2 lock() 0 3 spin(A) 0 4 unlock() 1 5 lock() 0 6 cmpxchg(1,0) 0 7 contended() -1 8 lock() 0 9 spin(C) 0 10 unlock() 1 11 cmpxchg(1,0) 0 12 unlock() 1 At this point, the lock is unlocked, but Task B is in an uninterruptible sleep with nobody to wake it up. This patch fixes the problem by ensuring we put the lock into the contended state if we fail to acquire it on the fastpath, ensuring that any blocked waiters are woken up when the mutex is released. Signed-off-by: Will Deacon <will.deacon@arm.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Chris Mason <chris.mason@fusionio.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: <stable@vger.kernel.org> Reviewed-by: Nicolas Pitre <nico@linaro.org> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Link: http://lkml.kernel.org/n/tip-6e9lrw2avczr0617fzl5vqb8@git.kernel.org Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Diffstat (limited to 'include/asm-generic')
-rw-r--r--include/asm-generic/mutex-xchg.h11
1 files changed, 9 insertions, 2 deletions
diff --git a/include/asm-generic/mutex-xchg.h b/include/asm-generic/mutex-xchg.h
index 580a6d3..c04e0db 100644
--- a/include/asm-generic/mutex-xchg.h
+++ b/include/asm-generic/mutex-xchg.h
@@ -26,7 +26,13 @@ static inline void
__mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
{
if (unlikely(atomic_xchg(count, 0) != 1))
- fail_fn(count);
+ /*
+ * We failed to acquire the lock, so mark it contended
+ * to ensure that any waiting tasks are woken up by the
+ * unlock slow path.
+ */
+ if (likely(atomic_xchg(count, -1) != 1))
+ fail_fn(count);
}
/**
@@ -43,7 +49,8 @@ static inline int
__mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *))
{
if (unlikely(atomic_xchg(count, 0) != 1))
- return fail_fn(count);
+ if (likely(atomic_xchg(count, -1) != 1))
+ return fail_fn(count);
return 0;
}