linux-kernel - Re: [PATCH] fs/xfs: Use atomic64_try_cmpxchg in xlog_grant_{add,sub}

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Message-ID: <20220809230244.GJ3600936@dread.disaster.area>
Date:   Wed, 10 Aug 2022 09:02:44 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Darrick J. Wong" <djwong@...nel.org>
Subject: Re: [PATCH] fs/xfs: Use atomic64_try_cmpxchg in
 xlog_grant_{add,sub}_space

On Wed, Aug 10, 2022 at 08:05:11AM +1000, Dave Chinner wrote:
> On Tue, Aug 09, 2022 at 06:56:15PM +0200, Uros Bizjak wrote:
> > Use `!atomic64_try_cmpxchg(ptr, &old, new)` instead of
> > `atomic64_cmpxchg(ptr, old, new) != old` in xlog_grant_{add,sub}_space.
> > This has two benefits:
> > 
> > - The x86 cmpxchg instruction returns success in the ZF flag, so this
> >   change saves a compare after cmpxchg, as well as a related move
> >   instruction in the front of cmpxchg.
> > 
> > - atomic64_try_cmpxchg implicitly assigns the *ptr value to &old when
> >   cmpxchg fails, enabling further code simplifications.
> 
> Do the two cmpxchg operations have the same memory ordering
> semantics on failure?
> 
> > This patch has no functional change.
> 
> The patch looks ok, but ....
> 
> ... I'm about 2 hours away from posting a patchset that completely
> removes the cmpxchg and the new grant head accounting has
> significantly lower fast path overhead. It also opens the door for
> tracking more than 2GB of log space in the grant heads.

FYI, the original RFC for this was posted a bit over a month ago:

https://lore.kernel.org/linux-xfs/20220708015558.1134330-1-david@fromorbit.com/

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com