From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f45.google.com (mail-pa0-f45.google.com [209.85.220.45]) by kanga.kvack.org (Postfix) with ESMTP id 7F3876B0005 for ; Wed, 9 Mar 2016 06:01:01 -0500 (EST) Received: by mail-pa0-f45.google.com with SMTP id tt10so36778389pab.3 for ; Wed, 09 Mar 2016 03:01:01 -0800 (PST) Received: from smtprelay.synopsys.com (smtprelay4.synopsys.com. [198.182.47.9]) by mx.google.com with ESMTPS id c67si11824851pfj.47.2016.03.09.03.01.00 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 09 Mar 2016 03:01:00 -0800 (PST) Subject: Re: [PATCH] mm: slub: Ensure that slab_unlock() is atomic 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> From: Vineet Gupta Message-ID: <56E0024F.4070401@synopsys.com> Date: Wed, 9 Mar 2016 16:30:31 +0530 MIME-Version: 1.0 In-Reply-To: <20160309101349.GJ6344@twins.programming.kicks-ass.net> Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Peter Zijlstra 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 On Wednesday 09 March 2016 03:43 PM, Peter Zijlstra wrote: >>> If you take the lock in __bit_spin_unlock >>> then the race cannot happen. >> >> Of course it won't but that means we penalize all non atomic callers of the API >> with a superfluous spinlock which is not require din first place given the >> definition of API. > > Quite. _However_, your arch is still broken, but not by your fault. Its > the generic-asm code that is wrong. > > The thing is that __bit_spin_unlock() uses __clear_bit_unlock(), which > defaults to __clear_bit(). Which is wrong. > > --- > 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. > > Reported-by: Vineet Gupta This needs to be CCed stable as it fixes a real bug for ARC. > Signed-off-by: Peter Zijlstra (Intel) Tested-by: Vineet Gupta FWIW, could we add some background to commit log, specifically what prompted this. Something like below... ---->8------ 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 ---->8------ > --- > 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. > */ To be able to override/use-generic don't we need #ifndef .... > #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_ */ > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org