From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932398AbcDJTM6 (ORCPT ); Sun, 10 Apr 2016 15:12:58 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:54099 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932820AbcDJTJO (ORCPT ); Sun, 10 Apr 2016 15:09:14 -0400 From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Vineet Gupta , "Peter Zijlstra (Intel)" , 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 Subject: [PATCH 4.4 167/210] bitops: Do not default to __clear_bit() for __clear_bit_unlock() Date: Sun, 10 Apr 2016 11:36:28 -0700 Message-Id: <20160410183532.569215472@linuxfoundation.org> X-Mailer: git-send-email 2.8.0 In-Reply-To: <20160410183526.651820045@linuxfoundation.org> References: <20160410183526.651820045@linuxfoundation.org> User-Agent: quilt/0.64 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 4.4-stable review patch. If anyone has any objections, please let me know. ------------------ 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: Greg Kroah-Hartman --- 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_ */