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

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Date:	Thu, 12 Feb 2015 14:50:56 +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 V3] x86 spinlock: Fix memory corruption on completing
	completions

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.

And perhaps you can turn these ACCESS_ONCE into READ_ONCE as well.

Oleg.

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