Message-ID: <54DCB761.4050504@linux.vnet.ibm.com>
Date: Thu, 12 Feb 2015 19:53:29 +0530
From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Organization: IBM
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To: Oleg Nesterov <oleg@redhat.com>
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Subject: Re: [PATCH V3] x86 spinlock: Fix memory corruption on completing
 completions
References: <1423741647-2156-1-git-send-email-raghavendra.kt@linux.vnet.ibm.com> <20150212135056.GA10646@redhat.com>
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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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