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

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Date:	Sun, 15 Feb 2015 17:17:33 +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 V5] x86 spinlock: Fix memory corruption on completing
	completions

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.



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

Oleg.

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