From mboxrd@z Thu Jan 1 00:00:00 1970 From: Raghavendra K T Subject: Re: [PATCH V5] x86 spinlock: Fix memory corruption on completing completions Date: Sun, 15 Feb 2015 11:31:54 +0530 Message-ID: <54E03652.8010104@linux.vnet.ibm.com> References: <1423979744-18320-1-git-send-email-raghavendra.kt@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1423979744-18320-1-git-send-email-raghavendra.kt@linux.vnet.ibm.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 To: sasha.levin@oracle.com Cc: jeremy@goop.org, Raghavendra K T , 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, davej@redhat.com, tglx@linutronix.de, waiman.long@hp.com, oleg@redhat.com, linux-kernel@vger.kernel.org, pbonzini@redhat.com, akpm@linux-foundation.org, torvalds@linux-foundation.org List-Id: virtualization@lists.linuxfoundation.org On 02/15/2015 11:25 AM, Raghavendra K T wrote: > Paravirt spinlock clears slowpath flag after doing unlock. > As explained by Linus currently it does: > prev = *lock; > add_smp(&lock->tickets.head, TICKET_LOCK_INC); > > /* add_smp() is a full mb() */ > > if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG)) > __ticket_unlock_slowpath(lock, prev); > > which is *exactly* the kind of things you cannot do with spinlocks, > because after you've done the "add_smp()" and released the spinlock > for the fast-path, you can't access the spinlock any more. Exactly > because a fast-path lock might come in, and release the whole data > structure. > > Linus suggested that we should not do any writes to lock after unlock(), > and we can move slowpath clearing to fastpath lock. > > So this patch implements the fix with: > 1. Moving slowpath flag to head (Oleg): > Unlocked locks don't care about the slowpath flag; therefore we can keep > it set after the last unlock, and clear it again on the first (try)lock. > -- this removes the write after unlock. note that keeping slowpath flag would > result in unnecessary kicks. > By moving the slowpath flag from the tail to the head ticket we also avoid > the need to access both the head and tail tickets on unlock. > > 2. use xadd to avoid read/write after unlock that checks the need for > unlock_kick (Linus): > We further avoid the need for a read-after-release by using xadd; > the prev head value will include the slowpath flag and indicate if we > need to do PV kicking of suspended spinners -- on modern chips xadd > isn't (much) more expensive than an add + load. > > Result: > setup: 16core (32 cpu +ht sandy bridge 8GB 16vcpu guest) > benchmark overcommit %improve > kernbench 1x -0.13 > kernbench 2x 0.02 > dbench 1x -1.77 > dbench 2x -0.63 > > [Jeremy: hinted missing TICKET_LOCK_INC for kick] > [Oleg: Moving slowpath flag to head, ticket_equals idea] > [PeterZ: Detailed changelog] > > Reported-by: Sasha Levin > Suggested-by: Linus Torvalds > Signed-off-by: Raghavendra K T > --- Sasha, Hope this addresses invalid read concern you had with latest xadd based implementation. (Think we need to test without Oleg's PATCH] sched/completion: completion_done() should serialize with complete() reported by Paul.)