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authorPeter Zijlstra <peterz@infradead.org>2016-03-09 12:40:54 +0100
committerIngo Molnar <mingo@kernel.org>2016-03-21 10:50:48 +0100
commitf75d48644c56a31731d17fa693c8175328957e1d (patch)
treed9851c52b67be05d80c69572873a18097c7b3e7b
parente525a8622a10f714c755038e07e86d06d880b8a2 (diff)
downloadlinux-f75d48644c56a31731d17fa693c8175328957e1d.tar.bz2
bitops: Do not default to __clear_bit() for __clear_bit_unlock()
__clear_bit_unlock() is a special little snowflake. While it carries the non-atomic '__' prefix, it is specifically documented to pair with test_and_set_bit() and therefore should be 'somewhat' atomic. Therefore the generic implementation of __clear_bit_unlock() cannot use the fully non-atomic __clear_bit() as a default. If an arch is able to do better; is must provide an implementation of __clear_bit_unlock() itself. Specifically, this came up as a result of hackbench livelock'ing in slab_lock() on ARC with SMP + SLUB + !LLSC. The issue was incorrect pairing of atomic ops. slab_lock() -> bit_spin_lock() -> test_and_set_bit() slab_unlock() -> __bit_spin_unlock() -> __clear_bit() The non serializing __clear_bit() was getting "lost" 80543b8e: ld_s r2,[r13,0] <--- (A) Finds PG_locked is set 80543b90: or r3,r2,1 <--- (B) other core unlocks right here 80543b94: st_s r3,[r13,0] <--- (C) sets PG_locked (overwrites unlock) Fixes ARC STAR 9000817404 (and probably more). Reported-by: Vineet Gupta <Vineet.Gupta1@synopsys.com> Tested-by: Vineet Gupta <Vineet.Gupta1@synopsys.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Christoph Lameter <cl@linux.com> Cc: David Rientjes <rientjes@google.com> Cc: Helge Deller <deller@gmx.de> Cc: James E.J. Bottomley <jejb@parisc-linux.org> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Noam Camus <noamc@ezchip.com> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Pekka Enberg <penberg@kernel.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: stable@vger.kernel.org Link: http://lkml.kernel.org/r/20160309114054.GJ6356@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
-rw-r--r--include/asm-generic/bitops/lock.h14
1 files changed, 7 insertions, 7 deletions
diff --git a/include/asm-generic/bitops/lock.h b/include/asm-generic/bitops/lock.h
index c30266e94806..8ef0ccbf8167 100644
--- a/include/asm-generic/bitops/lock.h
+++ b/include/asm-generic/bitops/lock.h
@@ -29,16 +29,16 @@ do { \
* @nr: the bit to set
* @addr: the address to start counting from
*
- * This operation is like clear_bit_unlock, however it is not atomic.
- * It does provide release barrier semantics so it can be used to unlock
- * a bit lock, however it would only be used if no other CPU can modify
- * any bits in the memory until the lock is released (a good example is
- * if the bit lock itself protects access to the other bits in the word).
+ * A weaker form of clear_bit_unlock() as used by __bit_lock_unlock(). If all
+ * the bits in the word are protected by this lock some archs can use weaker
+ * ops to safely unlock.
+ *
+ * See for example x86's implementation.
*/
#define __clear_bit_unlock(nr, addr) \
do { \
- smp_mb(); \
- __clear_bit(nr, addr); \
+ smp_mb__before_atomic(); \
+ clear_bit(nr, addr); \
} while (0)
#endif /* _ASM_GENERIC_BITOPS_LOCK_H_ */