linux-kernel - Re: [rfc] lock bitops

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Message-ID: <20070508223309.GB20174@wotan.suse.de>
Date:	Wed, 9 May 2007 00:33:09 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	David Howells <dhowells@...hat.com>
Cc:	linux-arch@...r.kernel.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rfc] lock bitops

On Tue, May 08, 2007 at 01:22:56PM +0100, David Howells wrote:
> Nick Piggin <npiggin@...e.de> wrote:
> 
> > This patch (along with the subsequent one to optimise unlock_page) reduces
> > the overhead of lock_page/unlock_page (measured with page faults and a patch
> > to lock the page in the fault handler) by about 425 cycles on my 2-way G5.
> 
> Seems reasonable, though test_and_set_lock_bit() might be a better name.

The postfix attaches lock semantics to the test_and_set_bit operation,
so I don't think it is necessarily wrong to have this name. But it doesn't
matter too much I guess.


> > +There are two special bitops with lock barrier semantics (acquire/release,
> > +same as spinlocks).
> 
> You should update Documentation/memory-barriers.txt also.

Will do.


> >  #define TestSetPageLocked(page)		\
> >  		test_and_set_bit(PG_locked, &(page)->flags)
> > +#define TestSetPageLocked_Lock(page)		\
> > +		test_and_set_bit_lock(PG_locked, &(page)->flags)
> 
> Can we get away with just moving TestSetPageLocked() to the new function
> rather than adding another accessor?  Or how about LockPageLocked() and
> UnlockPageLocked() rather than SetPageLocked_Lock() that last looks wrong
> somehow.

The problem is that it implies some semantics that it may not have. Possibly
better is to just remove them all and use only trylock/lock/unlock_page.
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