linux-kernel - Re: [PATCH 6/8] i386: bitops: Don't mark memory as clobbered unnecessarily

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Date:	Tue, 24 Jul 2007 17:55:12 -0400
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Satyam Sharma <ssatyam@....iitk.ac.in>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 6/8] i386: bitops: Don't mark memory as clobbered
	unnecessarily

On Wed, 2007-07-25 at 07:37 +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2007-07-24 at 11:13 -0700, Linus Torvalds wrote:
> > 
> > IOW, if you do a spinlock with the bitops, the locking side should be
> > able 
> > to use a "test_and_set_bit()" on its own, but the unlocking side
> > should be
> > 
> >         smp_mb__before_clear_bit();
> >         clear_bit();
> > 
> > because the ones that don't return a value also don't imply a memory 
> > barrier.
> 
> Yup. But I much prefer Nick's clear_bit_unlock() :-)
> 
> Ben

If you want to use bitops as spinlocks you should rather be using
<linux/bit_spinlock.h>. That also does the right thing w.r.t.
pre-emption and sparse locking annotations.

Trond

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