Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759488AbcDBBIi (ORCPT ); Fri, 1 Apr 2016 21:08:38 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:54532 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758882AbcDBA5D (ORCPT ); Fri, 1 Apr 2016 20:57:03 -0400 From: Kamal Mostafa To: linux-kernel@vger.kernel.org, stable@vger.kernel.org, kernel-team@lists.ubuntu.com Cc: Peter Zijlstra , Andrew Morton , Christoph Lameter , David Rientjes , Helge Deller , "James E . J . Bottomley" , Joonsoo Kim , Linus Torvalds , Noam Camus , "Paul E . McKenney" , Pekka Enberg , Thomas Gleixner , Ingo Molnar , Kamal Mostafa Subject: [PATCH 3.19.y-ckt 109/170] bitops: Do not default to __clear_bit() for __clear_bit_unlock() Date: Fri, 1 Apr 2016 17:53:15 -0700 Message-Id: <1459558456-24452-110-git-send-email-kamal@canonical.com> X-Mailer: git-send-email 2.7.4 In-Reply-To: <1459558456-24452-1-git-send-email-kamal@canonical.com> References: <1459558456-24452-1-git-send-email-kamal@canonical.com> X-Extended-Stable: 3.19 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3350 Lines: 86 3.19.8-ckt18 -stable review patch. If anyone has any objections, please let me know. ---8<------------------------------------------------------------ From: Peter Zijlstra 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 Tested-by: Vineet Gupta Signed-off-by: Peter Zijlstra (Intel) Cc: Andrew Morton Cc: Christoph Lameter Cc: David Rientjes Cc: Helge Deller Cc: James E.J. Bottomley Cc: Joonsoo Kim Cc: Linus Torvalds Cc: Noam Camus Cc: Paul E. McKenney Cc: Pekka Enberg Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/20160309114054.GJ6356@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar Signed-off-by: Kamal Mostafa --- 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