linux-kernel - Re: [PATCH V5] x86 spinlock: Fix memory corruption on completing completions

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Message-ID: <54E0D889.8030206@linux.vnet.ibm.com>
Date:	Sun, 15 Feb 2015 23:04:01 +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,
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	ak@...ux.intel.com, jasowang@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
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	a.ryabinin@...sung.com, sasha.levin@...cle.com, dave@...olabs.net
Subject: Re: [PATCH V5] x86 spinlock: Fix memory corruption on completing
 completions

On 02/15/2015 09:47 PM, Oleg Nesterov wrote:
> Well, I regret I mentioned the lack of barrier after enter_slowpath ;)
>
> On 02/15, Raghavendra K T wrote:
>>
>> @@ -46,7 +46,8 @@ static __always_inline bool static_key_false(struct static_key *key);
>>
>>   static inline void __ticket_enter_slowpath(arch_spinlock_t *lock)
>>   {
>> -	set_bit(0, (volatile unsigned long *)&lock->tickets.tail);
>> +	set_bit(0, (volatile unsigned long *)&lock->tickets.head);
>> +	barrier();
>>   }
>
> Because this barrier() looks really confusing.
>
> Firsty, it is equally unneeded on x86. At the same time, it can not help.
> We need a memory barrier() between set_bit(SLOWPATH) and READ_ONCE(head)
> to avoid the race with spin_unlock().
>
> So I think you should replace it with smp_mb__after_atomic() or remove it.
>

I resent the patch the above change.

>
> Other than that I believe this version is correct. So I won't insist, this
> is cosmetic after all.

Thanks Oleg.

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