linux-kernel - Re: [PATCH 10/11] qspinlock: Paravirt support

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Message-ID: <53A18000.8080909@redhat.com>
Date:	Wed, 18 Jun 2014 14:03:12 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Waiman Long <waiman.long@...com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	tglx@...utronix.de, mingo@...nel.org, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	paolo.bonzini@...il.com, konrad.wilk@...cle.com,
	boris.ostrovsky@...cle.com, paulmck@...ux.vnet.ibm.com,
	riel@...hat.com, torvalds@...ux-foundation.org,
	raghavendra.kt@...ux.vnet.ibm.com, david.vrabel@...rix.com,
	oleg@...hat.com, gleb@...hat.com, scott.norton@...com,
	chegu_vinod@...com, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 10/11] qspinlock: Paravirt support

Il 17/06/2014 00:08, Waiman Long ha scritto:
>> +void __pv_queue_unlock(struct qspinlock *lock)
>> +{
>> +	int val = atomic_read(&lock->val);
>> +
>> +	native_queue_unlock(lock);
>> +
>> +	if (val & _Q_LOCKED_SLOW)
>> +		___pv_kick_head(lock);
>> +}
>> +
>
> Again a race can happen here between the reading and writing of the lock
> value. I can't think of a good way to do that without using cmpxchg.

Could you just use xchg on the locked byte?

Paolo
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