linux-kernel - Re: [PATCH] locking/qrwlock: Allow multiple spinning readers

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Date:	Fri, 1 Apr 2016 12:29:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>,
	Will Deacon <will.deacon@....com>
Subject: Re: [PATCH] locking/qrwlock: Allow multiple spinning readers

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 ..