On Tue, Jul 07, 2015 at 03:30:22PM +0100, Waiman Long wrote: > 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. > > _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. It's probably best if I try to implement something and we can either pick holes in the patch or I'll realise why I'm wrong in the process :) Will -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
