Re: [PATCH 2/4] locking/qrwlock: Reduce reader/writer to reader lock transfer latency
From: Waiman Long
Date: Tue Jul 07 2015 - 10:30:40 EST
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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