From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932309AbcCILlJ (ORCPT ); Wed, 9 Mar 2016 06:41:09 -0500 Received: from casper.infradead.org ([85.118.1.10]:58630 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932179AbcCILlA (ORCPT ); Wed, 9 Mar 2016 06:41:00 -0500 Date: Wed, 9 Mar 2016 12:40:54 +0100 From: Peter Zijlstra To: Vineet Gupta Cc: "linux-arch@vger.kernel.org" , linux-parisc@vger.kernel, Andrew Morton , Helge Deller , linux-kernel@vger.kernel.org, stable@vger.kernel.org, "James E.J. Bottomley" , Pekka Enberg , linux-mm@kvack.org, Noam Camus , David Rientjes , Christoph Lameter , linux-snps-arc@lists.infradead.org, Joonsoo Kim Subject: Re: [PATCH] mm: slub: Ensure that slab_unlock() is atomic Message-ID: <20160309114054.GJ6356@twins.programming.kicks-ass.net> References: <1457447457-25878-1-git-send-email-vgupta@synopsys.com> <56DEF3D3.6080008@synopsys.com> <56DFC604.6070407@synopsys.com> <20160309101349.GJ6344@twins.programming.kicks-ass.net> <56E0024F.4070401@synopsys.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <56E0024F.4070401@synopsys.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 09, 2016 at 04:30:31PM +0530, Vineet Gupta wrote: > FWIW, could we add some background to commit log, specifically what prompted this. > Something like below... Sure.. find below. > > +++ b/include/asm-generic/bitops/lock.h > > @@ -29,16 +29,16 @@ do { \ > > * @nr: the bit to set > > * @addr: the address to start counting from > > * > > + * 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. > > */ > > To be able to override/use-generic don't we need #ifndef .... I did not follow through the maze, I think the few archs implementing this simply do not include this file at all. I'll let the first person that cares about this worry about that :-) --- Subject: 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). Cc: stable@vger.kernel.org Reported-by: Vineet Gupta Tested-by: Vineet Gupta Signed-off-by: Peter Zijlstra (Intel) --- 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 c30266e94806..8ef0ccbf8167 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_ */