linux-kernel - Re: [PATCH V4] x86 spinlock: Fix memory corruption on completing completions

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Date:	Sun, 15 Feb 2015 17:07:00 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.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 V4] x86 spinlock: Fix memory corruption on completing
	completions

On 02/15, Raghavendra K T wrote:
>
> On 02/13/2015 09:02 PM, Oleg Nesterov wrote:
>
>>> @@ -772,7 +773,8 @@ __visible void kvm_lock_spinning(struct arch_spinlock *lock, __ticket_t want)
>>>   	 * check again make sure it didn't become free while
>>>   	 * we weren't looking.
>>>   	 */
>>> -	if (ACCESS_ONCE(lock->tickets.head) == want) {
>>> +	head = READ_ONCE(lock->tickets.head);
>>> +	if (__tickets_equal(head, want)) {
>>>   		add_stats(TAKEN_SLOW_PICKUP, 1);
>>>   		goto out;
>>
>> This is off-topic, but with or without this change perhaps it makes sense
>> to add smp_mb__after_atomic(). It is nop on x86, just to make this code
>> more understandable for those (for me ;) who can never remember even the
>> x86 rules.
>
> Hope you meant it for add_stat.

No, no. We need a barrier between set_bit(SLOWPATH) and tickets_equal().

Yes, on x86 set_bit() can't be reordered so smp_mb_*_atomic() is nop, but
it can make the code more understandable.

> yes  smp_mb__after_atomic() would be
> harmless barrier() in x86. Did not add this V5 as yoiu though  but this
> made me look at slowpath_enter code and added an explicit barrier()
> there :).

Well. it looks even more confusing than a lack of barrier ;)

Oleg.

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