linux-kernel - Re: [PATCH 2/4] locking/qrwlock: Reduce reader/writer to reader lock transfer latency

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Date:	Tue, 7 Jul 2015 13:17:31 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>
Cc:	Waiman Long <waiman.long@...com>, Ingo Molnar <mingo@...hat.com>,
	Arnd Bergmann <arnd@...db.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH 2/4] locking/qrwlock: Reduce reader/writer to reader lock
 transfer latency

On Tue, Jul 07, 2015 at 10:17:11AM +0100, Will Deacon wrote:
> > > Thinking about it, can we kill _QW_WAITING altogether and set (cmpxchg
> > > from 0) wmode to _QW_LOCKED in the write_lock slowpath, polling (acquire)
> > > rmode until it hits zero?
> > 
> > No, this is how we make the lock fair so that an incoming streams of 
> > later readers won't block a writer from getting the lock.
> 
> But won't those readers effectively see that the lock is held for write
> (because we set wmode to _QW_LOCKED before the existing reader had drained)
> and therefore fall down the slow-path and get held up on the spinlock?

Yes, that's the entire point. Once there's a writer pending, new readers
should queue too.
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