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Date: Tue, 7 Jul 2015 12:49:18 +0100
From: Will Deacon <will.deacon@....com>
To: Peter Zijlstra <peterz@...radead.org>
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 12:17:31PM +0100, Peter Zijlstra wrote:
> 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.
Agreed. My point was that we can achieve the same result without
a separate _QW_WAITING flag afaict.
Will
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