Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754961AbcCULRy (ORCPT ); Mon, 21 Mar 2016 07:17:54 -0400 Received: from terminus.zytor.com ([198.137.202.10]:49582 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754760AbcCULRl (ORCPT ); Mon, 21 Mar 2016 07:17:41 -0400 Date: Mon, 21 Mar 2016 04:16:33 -0700 From: tip-bot for Peter Zijlstra Message-ID: Cc: rientjes@google.com, jejb@parisc-linux.org, mingo@kernel.org, deller@gmx.de, cl@linux.com, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, Vineet.Gupta1@synopsys.com, peterz@infradead.org, iamjoonsoo.kim@lge.com, hpa@zytor.com, tglx@linutronix.de, penberg@kernel.org, torvalds@linux-foundation.org, paulmck@linux.vnet.ibm.com, noamc@ezchip.com Reply-To: penberg@kernel.org, torvalds@linux-foundation.org, paulmck@linux.vnet.ibm.com, noamc@ezchip.com, mingo@kernel.org, rientjes@google.com, jejb@parisc-linux.org, deller@gmx.de, linux-kernel@vger.kernel.org, cl@linux.com, akpm@linux-foundation.org, tglx@linutronix.de, Vineet.Gupta1@synopsys.com, peterz@infradead.org, hpa@zytor.com, iamjoonsoo.kim@lge.com In-Reply-To: <20160309114054.GJ6356@twins.programming.kicks-ass.net> References: <20160309114054.GJ6356@twins.programming.kicks-ass.net> To: linux-tip-commits@vger.kernel.org Subject: [tip:locking/urgent] bitops: Do not default to __clear_bit() for __clear_bit_unlock() Git-Commit-ID: f75d48644c56a31731d17fa693c8175328957e1d X-Mailer: tip-git-log-daemon Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3437 Lines: 85 Commit-ID: f75d48644c56a31731d17fa693c8175328957e1d Gitweb: http://git.kernel.org/tip/f75d48644c56a31731d17fa693c8175328957e1d Author: Peter Zijlstra AuthorDate: Wed, 9 Mar 2016 12:40:54 +0100 Committer: Ingo Molnar CommitDate: Mon, 21 Mar 2016 10:50:48 +0100 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 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 Cc: stable@vger.kernel.org Link: http://lkml.kernel.org/r/20160309114054.GJ6356@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar --- 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_ */