3.16.35-rc1 review patch. If anyone has any objections, please let me know.
------------------ From: Peter Zijlstra <pet...@infradead.org> 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 <vineet.gup...@synopsys.com> Tested-by: Vineet Gupta <vineet.gup...@synopsys.com> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> Cc: Andrew Morton <a...@linux-foundation.org> Cc: Christoph Lameter <c...@linux.com> Cc: David Rientjes <rient...@google.com> Cc: Helge Deller <del...@gmx.de> Cc: James E.J. Bottomley <j...@parisc-linux.org> Cc: Joonsoo Kim <iamjoonsoo....@lge.com> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: Noam Camus <no...@ezchip.com> Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com> Cc: Pekka Enberg <penb...@kernel.org> Cc: Peter Zijlstra <pet...@infradead.org> Cc: Thomas Gleixner <t...@linutronix.de> Link: http://lkml.kernel.org/r/20160309114054.gj6...@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar <mi...@kernel.org> Signed-off-by: Ben Hutchings <b...@decadent.org.uk> --- include/asm-generic/bitops/lock.h | 14 +++++++------- 1 file changed, 7 insertions(+), 7 deletions(-) --- 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_ */
