From mboxrd@z Thu Jan 1 00:00:00 1970 From: Raghavendra K T Subject: Re: [PATCH V3] x86 spinlock: Fix memory corruption on completing completions Date: Thu, 12 Feb 2015 19:53:29 +0530 Message-ID: <54DCB761.4050504@linux.vnet.ibm.com> References: <1423741647-2156-1-git-send-email-raghavendra.kt@linux.vnet.ibm.com> <20150212135056.GA10646@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Cc: jeremy@goop.org, kvm@vger.kernel.org, peterz@infradead.org, virtualization@lists.linux-foundation.org, paul.gortmaker@windriver.com, hpa@zytor.com, ak@linux.intel.com, a.ryabinin@samsung.com, x86@kernel.org, borntraeger@de.ibm.com, mingo@redhat.com, xen-devel@lists.xenproject.org, paulmck@linux.vnet.ibm.com, riel@redhat.com, konrad.wilk@oracle.com, dave@stgolabs.net, sasha.levin@oracle.com, davej@redhat.com, tglx@linutronix.de, waiman.long@hp.com, linux-kernel@vger.kernel.org, pbonzini@redhat.com, akpm@linux-foundation.org, torvalds@linux-foundation.org To: Oleg Nesterov Return-path: In-Reply-To: <20150212135056.GA10646@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org List-Id: kvm.vger.kernel.org On 02/12/2015 07:20 PM, Oleg Nesterov wrote: > On 02/12, Raghavendra K T wrote: >> >> @@ -191,8 +189,7 @@ static inline void arch_spin_unlock_wait(arch_spinlock_t *lock) >> * We need to check "unlocked" in a loop, tmp.head == head >> * can be false positive because of overflow. >> */ >> - if (tmp.head == (tmp.tail & ~TICKET_SLOWPATH_FLAG) || >> - tmp.head != head) >> + if (__tickets_equal(tmp.head, tmp.tail) || tmp.head != head) >> break; > > Ah, it seems that "tmp.head != head" should be turned into > !__tickets_equal(), no? > > Suppose that TICKET_SLOWPATH_FLAG is set after the first ACCESS_ONCE(head), > then tmp.head != head will be true before the first unlock we are waiting > for. Good catch. othewise we would wrongly break out even when somebody does halt wait. > > And perhaps you can turn these ACCESS_ONCE into READ_ONCE as well. Yes again :)