linux-kernel - Re: [rfc] optimise unlock

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Date:	Thu, 10 May 2007 07:21:30 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Nick Piggin <npiggin@...e.de>, linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [rfc] optimise unlock_page


> Not good enough, I'm afraid.  It looks like Ben's right and you need
> a count - and counts in the page struct are a lot harder to add than
> page flags.
> 
> I've now played around with the hangs on my three 4CPU machines
> (all of them in io_schedule below __lock_page, waiting on pages
> which were neither PG_locked nor PG_waiters when I looked).
> 
> Seeing Ben's mail, I thought the answer would be just to remove
> the "_exclusive" from your three prepare_to_wait_exclusive()s.
> That helped, but it didn't eliminate the hangs.

There might be a way ... by having the flags manipulation always
atomically deal with PG_locked and PG_waiters together. This is possible
but we would need even more weirdo bitops abstractions from the arch I'm
afraid... unless we start using atomic_* rather that bitops in order to
manipulate multiple bits at a time.

Ben.


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