Date: Fri, 1 Apr 2016 12:29:30 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hpe.com>
Cc: Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        Scott J Norton <scott.norton@hpe.com>,
        Douglas Hatch <doug.hatch@hpe.com>, Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH] locking/qrwlock: Allow multiple spinning readers
Message-ID: <20160401102930.GI3448@twins.programming.kicks-ass.net>
References: <1458444079-59601-1-git-send-email-Waiman.Long@hpe.com>
 <20160329202050.GN3408@twins.programming.kicks-ass.net>
 <56FDA0D6.4090904@hpe.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <56FDA0D6.4090904@hpe.com>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1221
Lines: 30

On Thu, Mar 31, 2016 at 06:12:38PM -0400, Waiman Long wrote:
> On 03/29/2016 04:20 PM, Peter Zijlstra wrote:
> >>  	cnts = atomic_add_return_acquire(_QR_BIAS,&lock->cnts) - _QR_BIAS;
> >>+	while ((cnts&  _QW_WMASK) == _QW_LOCKED) {
> >>+		if (locked&&  ((cnts>>  _QR_SHIFT)<  MAX_SPINNING_READERS)) {
> >>+			/*
> >>+			 * Unlock the wait queue so that more readers can
> >>+			 * come forward and waiting for the writer to exit
> >>+			 * as long as no more than MAX_SPINNING_READERS
> >>+			 * readers are present.
> >>+			 */
> >>+			arch_spin_unlock(&lock->wait_lock);
> >>+			locked = false;
> >Only 1 more can come forward with this logic. How can you ever get to 4?
> 
> Yes, each reader in the unlock path will release one in the queue. If the
> next one is also a reader, it will release one more and so on until the
> reader count reach 4 where the process will stop.
> 
> >
> >Also, what says the next in queue is a reader?
> 
> I did say in the changelog that the queue head could be a writer. In that
> case, the process will stop and the writer will wait until all the readers
> are gone.

Clearly I could not read not think straight when I looked at this patch.
You're right.

I'll go have another look ..