linux-kernel - Re: [PATCH V2] x86 spinlock: Fix memory corruption on completing completions

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Date:	Mon, 9 Feb 2015 16:14:59 +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
Subject: Re: [PATCH V2] x86 spinlock: Fix memory corruption on completing
	completions

On 02/09, Raghavendra K T wrote:
>
> +static inline void __ticket_check_and_clear_slowpath(arch_spinlock_t *lock)
> +{
> +	arch_spinlock_t old, new;
> +	__ticket_t diff;
> +
> +	old.tickets = READ_ONCE(lock->tickets);
> +	diff = (old.tickets.tail & ~TICKET_SLOWPATH_FLAG) - old.tickets.head;
> +
> +	/* try to clear slowpath flag when there are no contenders */
> +	if ((old.tickets.tail & TICKET_SLOWPATH_FLAG) &&
> +		(diff == TICKET_LOCK_INC)) {
> +		new = old;
> +		new.tickets.tail &= ~TICKET_SLOWPATH_FLAG;
> +		cmpxchg(&lock->head_tail, old.head_tail, new.head_tail);
> +	}
> +}

Can't we simplify it? We own .head, and we already know it. We only need
to clear TICKET_SLOWPATH_FLAG in .tail atomically?

IOW,

	static inline void __ticket_check_and_clear_slowpath(arch_spinlock_t *lock, __ticket_t head)
	{
		__ticket_t old_tail, new_tail;

		new_tail = head + TICKET_LOCK_INC;
		old_tail = new_tail | TICKET_SLOWPATH_FLAG;

		if (READ_ONCE(lock->tickets.tail) == old_tail)
			cmpxchg(&lock->tickets.tail, old_tail, new_tail);
	}

Plus

	-	__ticket_check_and_clear_slowpath(lock);
	+	__ticket_check_and_clear_slowpath(lock, inc.tail);

Or I missed something?

And I think it would be better to avoid ifdef(CONFIG_PARAVIRT_SPINLOCKS),
ww can just do

	if (TICKET_SLOWPATH_FLAG)
		__ticket_check_and_clear_slowpath();

Oleg.

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