From patchwork Wed Mar 9 11:40:54 2016 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Peter Zijlstra X-Patchwork-Id: 594996 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.9]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by ozlabs.org (Postfix) with ESMTPS id ADA04140BAC for ; Wed, 9 Mar 2016 22:41:02 +1100 (AEDT) Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.80.1 #2 (Red Hat Linux)) id 1adcU9-0000S3-De; Wed, 09 Mar 2016 11:41:01 +0000 Received: from casper.infradead.org ([2001:770:15f::2]) by bombadil.infradead.org with esmtps (Exim 4.80.1 #2 (Red Hat Linux)) id 1adcU8-0000Rc-6I for linux-snps-arc@bombadil.infradead.org; Wed, 09 Mar 2016 11:41:00 +0000 Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=twins) by casper.infradead.org with esmtpsa (Exim 4.85 #2 (Red Hat Linux)) id 1adcU3-0007cQ-DO; Wed, 09 Mar 2016 11:40:55 +0000 Received: by twins (Postfix, from userid 1000) id 7159510019BC7; Wed, 9 Mar 2016 12:40:54 +0100 (CET) Date: Wed, 9 Mar 2016 12:40:54 +0100 From: Peter Zijlstra To: Vineet Gupta 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-Disposition: inline In-Reply-To: <56E0024F.4070401@synopsys.com> User-Agent: Mutt/1.5.21 (2012-12-30) X-BeenThere: linux-snps-arc@lists.infradead.org X-Mailman-Version: 2.1.20 Precedence: list List-Id: Linux on Synopsys ARC Processors List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "linux-arch@vger.kernel.org" , linux-parisc@vger.kernel, 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 , Joonsoo Kim , Andrew Morton , linux-snps-arc@lists.infradead.org, Christoph Lameter Sender: "linux-snps-arc" Errors-To: linux-snps-arc-bounces+incoming=patchwork.ozlabs.org@lists.infradead.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_ */