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

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Date:	Tue, 07 Jul 2015 10:30:22 -0400
From:	Waiman Long <waiman.long@...com>
To:	Will Deacon <will.deacon@....com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	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 07/07/2015 07:49 AM, Will Deacon wrote:
> 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
>

_QW_WAITING and _QW_LOCKED has different semantics and are necessary for 
the proper handshake between readers and writer. We set _QW_WAITING when 
readers own the lock and the writer is waiting for the readers to go 
away. The _QW_WAITING flag will force new readers to go to queuing while 
the writer is waiting. We set _QW_LOCKED when a writer own the lock and 
it can only be set atomically when no reader is present. Without the 
intermediate _QW_WAITING step, a continuous stream of incoming readers 
(which make the reader count never 0) could deny a writer from getting 
the lock indefinitely.

Cheers,
Longman
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