4.2.8-ckt7 -stable review patch. If anyone has any objections, please let me know.
---8<------------------------------------------------------------ From: Peter Zijlstra <[email protected]> commit f75d48644c56a31731d17fa693c8175328957e1d upstream. __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 <[email protected]> Tested-by: Vineet Gupta <[email protected]> Signed-off-by: Peter Zijlstra (Intel) <[email protected]> Cc: Andrew Morton <[email protected]> Cc: Christoph Lameter <[email protected]> Cc: David Rientjes <[email protected]> Cc: Helge Deller <[email protected]> Cc: James E.J. Bottomley <[email protected]> Cc: Joonsoo Kim <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Noam Camus <[email protected]> Cc: Paul E. McKenney <[email protected]> Cc: Pekka Enberg <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Thomas Gleixner <[email protected]> Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: Ingo Molnar <[email protected]> Signed-off-by: Kamal Mostafa <[email protected]> --- include/asm-generic/bitops/lock.h | 14 +++++++------- 1 file changed, 7 insertions(+), 7 deletions(-) diff --git a/include/asm-generic/bitops/lock.h b/include/asm-generic/bitops/lock.h index c30266e..8ef0ccb 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_ */ -- 2.7.4
