From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752857AbeBSOBi (ORCPT ); Mon, 19 Feb 2018 09:01:38 -0500 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:59478 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752686AbeBSOBf (ORCPT ); Mon, 19 Feb 2018 09:01:35 -0500 Date: Mon, 19 Feb 2018 14:01:43 +0000 From: Will Deacon To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, mingo@kernel.org Subject: Re: [RFC PATCH 3/5] asm-generic/bitops/atomic.h: Rewrite using atomic_fetch_* Message-ID: <20180219140142.GD30394@arm.com> References: <1518708575-12284-1-git-send-email-will.deacon@arm.com> <1518708575-12284-4-git-send-email-will.deacon@arm.com> <20180215170847.GD25181@hirez.programming.kicks-ass.net> <20180215182049.GC15274@arm.com> <20180216102100.GB25201@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180216102100.GB25201@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Peter, On Fri, Feb 16, 2018 at 11:21:00AM +0100, Peter Zijlstra wrote: > On Thu, Feb 15, 2018 at 06:20:49PM +0000, Will Deacon wrote: > > On Thu, Feb 15, 2018 at 06:08:47PM +0100, Peter Zijlstra wrote: > > > On Thu, Feb 15, 2018 at 03:29:33PM +0000, Will Deacon wrote: > > > > +static inline void __clear_bit_unlock(unsigned int nr, > > > > + volatile unsigned long *p) > > > > +{ > > > > + unsigned long old; > > > > > > > > + p += BIT_WORD(nr); > > > > + old = READ_ONCE(*p); > > > > + old &= ~BIT_MASK(nr); > > > > + smp_store_release(p, old); > > > > > > This should be atomic_set_release() I think, for the special case where > > > atomics are implemented with spinlocks, see the 'fun' case in > > > Documentation/atomic_t.txt. > > > > My understanding of __clear_bit_unlock is that there is guaranteed to be > > no concurrent accesses to the same word, so why would it matter whether > > locks are used to implement atomics? > > > commit f75d48644c56a31731d17fa693c8175328957e1d > Author: Peter Zijlstra > Date: Wed Mar 9 12:40:54 2016 +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) Ah, so it's problematic for the case where atomics are built using locks. Got it. I'll err on the side of caution here and have the asm-generic header (which should be bitops/lock.h not bitops/atomic.h) conditionally define __clear_bit_unlock as clear_bit_lock unless the architecture has provided its own implementation. Thanks, Will