linux-kernel - Re: [PATCH 02/10] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath

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Message-ID: <20180406150819.GB10528@arm.com>
Date:   Fri, 6 Apr 2018 16:08:19 +0100
From:   Will Deacon <will.deacon@....com>
To:     Waiman Long <longman@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        peterz@...radead.org, mingo@...nel.org, boqun.feng@...il.com,
        paulmck@...ux.vnet.ibm.com, catalin.marinas@....com
Subject: Re: [PATCH 02/10] locking/qspinlock: Remove unbounded cmpxchg loop
 from locking slowpath

On Thu, Apr 05, 2018 at 05:16:16PM -0400, Waiman Long wrote:
> On 04/05/2018 12:58 PM, Will Deacon wrote:
> >  	/*
> > -	 * we're pending, wait for the owner to go away.
> > -	 *
> > -	 * *,1,1 -> *,1,0
> > -	 *
> > -	 * this wait loop must be a load-acquire such that we match the
> > -	 * store-release that clears the locked bit and create lock
> > -	 * sequentiality; this is because not all clear_pending_set_locked()
> > -	 * implementations imply full barriers.
> > -	 */
> > -	smp_cond_load_acquire(&lock->val.counter, !(VAL & _Q_LOCKED_MASK));
> > -
> > -	/*
> > -	 * take ownership and clear the pending bit.
> > -	 *
> > -	 * *,1,0 -> *,0,1
> > +	 * If pending was clear but there are waiters in the queue, then
> > +	 * we need to undo our setting of pending before we queue ourselves.
> >  	 */
> > -	clear_pending_set_locked(lock);
> > -	return;
> > +	if (!(val & _Q_PENDING_MASK))
> > +		atomic_andnot(_Q_PENDING_VAL, &lock->val);
> Can we add a clear_pending() helper that will just clear the byte if
> _Q_PENDING_BITS == 8? That will eliminate one atomic instruction from
> the failure path.

Good idea!

Will