From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christian Borntraeger Subject: Re: [PATCHv2 04/10] x86/spinlock: Replace ACCESS_ONCE with READ_ONCE/ASSIGN_ONCE Date: Tue, 25 Nov 2014 21:29:01 +0100 Message-ID: <5474E68D.4010707@de.ibm.com> References: <1416919117-50652-1-git-send-email-borntraeger@de.ibm.com> <1416919117-50652-5-git-send-email-borntraeger@de.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Return-path: Received: from e06smtp11.uk.ibm.com ([195.75.94.107]:48053 "EHLO e06smtp11.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751092AbaKYU3J (ORCPT ); Tue, 25 Nov 2014 15:29:09 -0500 Received: from /spool/local by e06smtp11.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 25 Nov 2014 20:29:07 -0000 In-Reply-To: <1416919117-50652-5-git-send-email-borntraeger@de.ibm.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: linux-kernel@vger.kernel.org Cc: linux-arch@vger.kernel.org, linux-mips@linux-mips.org, linux-x86_64@vger.kernel.org, linux-s390@vger.kernel.org, Paolo Bonzini , paulmck@linux.vnet.ibm.com, mingo@kernel.org, torvalds@linux-foundation.org, Catalin Marinas , Will Deacon , Alexei Starovoitov , David Howells , Russell King Am 25.11.2014 um 13:38 schrieb Christian Borntraeger: > ACCESS_ONCE does not work reliably on non-scalar types. For > example gcc 4.6 and 4.7 might remove the volatile tag for such > accesses during the SRA (scalar replacement of aggregates) step > (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145) > > Change the spinlock code to replace ACCESS_ONCE with READ_ONCE > and ASSIGN_ONCE. > > Signed-off-by: Christian Borntraeger > --- > arch/x86/include/asm/spinlock.h | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h > index 9295016..af6e673 100644 > --- a/arch/x86/include/asm/spinlock.h > +++ b/arch/x86/include/asm/spinlock.h > @@ -92,7 +92,7 @@ static __always_inline void arch_spin_lock(arch_spinlock_t *lock) > unsigned count = SPIN_THRESHOLD; > > do { > - if (ACCESS_ONCE(lock->tickets.head) == inc.tail) > + if (ASSIGN_ONCE(inc.tail, lock->tickets.head)) As Mike pointed out: this should be if (READ_ONCE(lock->tickets.head) == inc.tail) of course.