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

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Date:	Thu, 12 Feb 2015 15:02:48 +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

Damn, sorry for noise, forgot to mention...

On 02/12, Raghavendra K T wrote:
>
> +static inline void __ticket_check_and_clear_slowpath(arch_spinlock_t *lock,
> +							__ticket_t head)
> +{
> +	if (head & TICKET_SLOWPATH_FLAG) {
> +		arch_spinlock_t old, new;
> +
> +		old.tickets.head = head;
> +		new.tickets.head = head & ~TICKET_SLOWPATH_FLAG;
> +		old.tickets.tail = new.tickets.head + TICKET_LOCK_INC;
> +		new.tickets.tail = old.tickets.tail;
> +
> +		/* try to clear slowpath flag when there are no contenders */
> +		cmpxchg(&lock->head_tail, old.head_tail, new.head_tail);
> +	}
> +}

...

> +clear_slowpath:
> +	if (TICKET_SLOWPATH_FLAG)
> +		__ticket_check_and_clear_slowpath(lock, inc.head);

I think you can remove this "if (TICKET_SLOWPATH_FLAG)" check. If it is
defined as zero, gcc should optimize out the code under "if (head & 0)".

But this is purely cosmetic and subjective, feel free to ignore.

Oleg.

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