From mboxrd@z Thu Jan  1 00:00:00 1970
From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Subject: Re: [PATCH] x86 spinlock: Fix memory corruption on completing completions
Date: Wed, 11 Feb 2015 16:38:58 +0530
Message-ID: <54DB384A.2050305@linux.vnet.ibm.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> <CA+55aFympdPOotzEgdhQULidN+nxb-VUdfym+qU9LOsHScvpzw@mail.gmail.com> <54D9CFC7.5020007@linux.vnet.ibm.com> <20150210132634.GA30380@redhat.com>
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Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Jeremy Fitzhardinge <jeremy@goop.org>,
	Sasha Levin <sasha.levin@oracle.com>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Peter Anvin <hpa@zytor.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	Waiman Long <waiman.long@hp.com>,
	Dave Jones <davej@redhat.com>,
	the arch/x86 maintainers <x86@kernel.org>,
	Paul Gortmaker <paul.gortmaker@windriver.com>,
	Andi Kleen <ak@linux.intel.com>,
	Jason Wang <jasowang@redhat.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	KVM list <kvm@vger.kernel.org>,
	virtualization <virtualization@lists.linux-foundation.org>,
	xen-devel@lists.xenproject.org, Rik van Riel <riel@redhat.com>,
	Christian B
To: Oleg Nesterov <oleg@redhat.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <20150210132634.GA30380@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: kvm.vger.kernel.org

On 02/10/2015 06:56 PM, Oleg Nesterov wrote:
> 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

It is a good idea. Trying this now.

> 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.

Correct, but apart from this, before doing xadd in unlock,
we would have to make sure lsb bit is cleared so that we can live with 1 
bit overflow to tail which is unused. now either or both of head,tail
lsb bit may be set after unlock.