linux-kernel - Re: [PATCH 8/9] qspinlock: Generic paravirt support

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Date:	Thu, 19 Mar 2015 14:43:36 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@...com>
Cc:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	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, scott.norton@...com, doug.hatch@...com,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	luto@...capital.net
Subject: Re: [PATCH 8/9] qspinlock: Generic paravirt support

On Thu, Mar 19, 2015 at 01:25:36PM +0100, Peter Zijlstra wrote:
> +static struct qspinlock **pv_hash(struct qspinlock *lock)
> +{
> +	u32 hash = hash_ptr(lock, PV_LOCK_HASH_BITS);
> +	struct pv_hash_bucket *hb, *end;
> +
> +	if (!hash)
> +		hash = 1;
> +
> +	hb = &__pv_lock_hash[hash_align(hash)];
> +	for (;;) {
> +		for (end = hb + PV_HB_PER_LINE; hb < end; hb++) {
> +			if (cmpxchg(&hb->lock, NULL, HB_RESERVED)) {

That should be: !cmpxchg(), bit disturbing that that booted.

> +				WRITE_ONCE(hb->cpu, smp_processor_id());
> +				/*
> +				 * Since we must read lock first and cpu
> +				 * second, we must write cpu first and lock
> +				 * second, therefore use HB_RESERVE to mark an
> +				 * entry in use before writing the values.
> +				 *
> +				 * This can cause hb_hash_find() to not find a
> +				 * cpu even though _Q_SLOW_VAL, this is not a
> +				 * problem since we re-check l->locked before
> +				 * going to sleep and the unlock will have
> +				 * cleared l->locked already.
> +				 */
> +				smp_wmb(); /* matches rmb from pv_hash_find */
> +				WRITE_ONCE(hb->lock, lock);
> +				goto done;
> +			}
> +		}
> +
> +		hash = lfsr(hash, PV_LOCK_HASH_BITS);
> +		hb = &__pv_lock_hash[hash_align(hash)];
> +	}
> +
> +done:
> +	return &hb->lock;
> +}
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