Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755220AbbBJN24 (ORCPT ); Tue, 10 Feb 2015 08:28:56 -0500 Received: from mx1.redhat.com ([209.132.183.28]:40313 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751973AbbBJN2y (ORCPT ); Tue, 10 Feb 2015 08:28:54 -0500 Date: Tue, 10 Feb 2015 14:26:34 +0100 From: Oleg Nesterov To: Raghavendra K T Cc: Linus Torvalds , Jeremy Fitzhardinge , Sasha Levin , Davidlohr Bueso , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Peter Anvin , Konrad Rzeszutek Wilk , Paolo Bonzini , Paul McKenney , Waiman Long , Dave Jones , the arch/x86 maintainers , Paul Gortmaker , Andi Kleen , Jason Wang , Linux Kernel Mailing List , KVM list , virtualization , xen-devel@lists.xenproject.org, Rik van Riel , Christian Borntraeger , Andrew Morton , Andrey Ryabinin Subject: Re: [PATCH] x86 spinlock: Fix memory corruption on completing completions Message-ID: <20150210132634.GA30380@redhat.com> References: <1423234148-13886-1-git-send-email-raghavendra.kt@linux.vnet.ibm.com> <54D7D19B.1000103@goop.org> <54D87F1E.9060307@linux.vnet.ibm.com> <20150209120227.GT21418@twins.programming.kicks-ass.net> <54D9CFC7.5020007@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <54D9CFC7.5020007@linux.vnet.ibm.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2141 Lines: 56 On 02/10, Raghavendra K T wrote: > > On 02/10/2015 06:23 AM, Linus Torvalds wrote: > >> add_smp(&lock->tickets.head, TICKET_LOCK_INC); >> if (READ_ONCE(lock->tickets.tail) & TICKET_SLOWPATH_FLAG) .. >> >> into something like >> >> val = xadd((&lock->ticket.head_tail, TICKET_LOCK_INC << TICKET_SHIFT); >> if (unlikely(val & TICKET_SLOWPATH_FLAG)) ... >> >> would be the right thing to do. Somebody should just check that I got >> that shift right, and that the tail is in the high bytes (head really >> needs to be high to work, if it's in the low byte(s) the xadd would >> overflow from head into tail which would be wrong). > > Unfortunately xadd could result in head overflow as tail is high. > > The other option was repeated cmpxchg which is bad I believe. > Any suggestions? Stupid question... what if we simply move SLOWPATH from .tail to .head? In this case arch_spin_unlock() could do xadd(tickets.head) and check the result In this case __ticket_check_and_clear_slowpath() really needs to cmpxchg the whole .head_tail. Plus obviously more boring changes. This needs a separate patch even _if_ this can work. BTW. If we move "clear slowpath" into "lock" path, then probably trylock should be changed too? Something like below, we just need to clear SLOWPATH before cmpxchg. Oleg. --- x/arch/x86/include/asm/spinlock.h +++ x/arch/x86/include/asm/spinlock.h @@ -109,7 +109,8 @@ static __always_inline int arch_spin_try if (old.tickets.head != (old.tickets.tail & ~TICKET_SLOWPATH_FLAG)) return 0; - new.head_tail = old.head_tail + (TICKET_LOCK_INC << TICKET_SHIFT); + new.tickets.head = old.tickets.head; + new.tickets.tail = (old.tickets.tail & ~TICKET_SLOWPATH_FLAG) + TICKET_LOCK_INC; /* cmpxchg is a full barrier, so nothing can move before it */ return cmpxchg(&lock->head_tail, old.head_tail, new.head_tail) == old.head_tail; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/