linux-kernel - Re: [PATCH] x86 spinlock: Fix memory corruption on completing completions

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Message-ID: <54DBA199.8070708@linux.vnet.ibm.com>
Date:	Thu, 12 Feb 2015 00:08:17 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Peter Anvin <hpa@...or.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Waiman Long <waiman.long@...com>,
	Dave Jones <davej@...hat.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Jason Wang <jasowang@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	virtualization <virtualization@...ts.linux-foundation.org>,
	xen-devel@...ts.xenproject.org, Rik van Riel <riel@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrey Ryabinin <a.ryabinin@...sung.com>
Subject: Re: [PATCH] x86 spinlock: Fix memory corruption on completing completions

On 02/11/2015 11:08 PM, Oleg Nesterov wrote:
> On 02/11, Raghavendra K T wrote:
>>
>> On 02/10/2015 06:56 PM, Oleg Nesterov wrote:
>>
>>> 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.
>
> Sorry, can't understand... could you spell?
>
> If TICKET_SLOWPATH_FLAG lives in .head arch_spin_unlock() could simply do
>
> 	head = xadd(&lock->tickets.head, TICKET_LOCK_INC);
>
> 	if (head & TICKET_SLOWPATH_FLAG)
> 		__ticket_unlock_kick(head);
>
> so it can't overflow to .tail?
>

You are right.
I totally forgot we can get rid of tail operations :)

>
> And we we do this, probably it makes sense to add something like
>
> 	bool tickets_equal(__ticket_t one, __ticket_t two)
> 	{
> 		return (one ^ two) & ~TICKET_SLOWPATH_FLAG;
> 	}
>

Very nice idea. I was tired of ~TICKET_SLOWPATH_FLAG usage all over in
the current (complex :)) implementation. These two suggestions helps
alot.

> and change kvm_lock_spinning() to use tickets_equal(tickets.head, want), plus
> it can have more users in asm/spinlock.h.

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