linux-kernel - Re: [PATCH V3] x86 spinlock: Fix memory corruption on completing completions

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Date:	Thu, 12 Feb 2015 19:53:29 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	peterz@...radead.org, torvalds@...ux-foundation.org,
	konrad.wilk@...cle.com, pbonzini@...hat.com,
	paulmck@...ux.vnet.ibm.com, waiman.long@...com, davej@...hat.com,
	x86@...nel.org, jeremy@...p.org, paul.gortmaker@...driver.com,
	ak@...ux.intel.com, jasowang@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, riel@...hat.com,
	borntraeger@...ibm.com, akpm@...ux-foundation.org,
	a.ryabinin@...sung.com, sasha.levin@...cle.com, dave@...olabs.net
Subject: Re: [PATCH V3] x86 spinlock: Fix memory corruption on completing
 completions

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 :)

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