linux-kernel - Re: [PATCH] block: Use try

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Message-ID: <CAFULd4bqmbundK=YE9A1scW6za3z4B7B_XjY=sS1R+3H40fF0Q@mail.gmail.com>
Date:   Tue, 12 Jul 2022 09:04:52 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-block@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] block: Use try_cmpxchg some more

On Tue, Jul 12, 2022 at 7:13 AM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Mon, Jul 11, 2022 at 05:33:01PM +0200, Uros Bizjak wrote:
> > Use try_cmpxchg family of functions instead of cmpxchg (*ptr, old, new) == old.
> > x86 CMPXCHG instruction returns success in ZF flag, so this change saves a
> > compare after cmpxchg (and related move instruction in front of cmpxchg).
> >
> > Also, try_cmpxchg implicitly assigns old *ptr value to "old" when
> > cmpxchg fails, enabling further code simplifications.
> >
> > No functional change intended.
>
> You might want to split this into a patch per caller as it might
> attact different reviewers.

No problem for me. get_maintainer.pl returned Jens as the sole
maintainer for all these parts, so I put everything together in order
to ease the maintainer's job.

> > +     do {
> > +     } while (old && !atomic_try_cmpxchg(&blkg->use_delay, &old, old - 1));
>
> It might just be me, but for loops with an empty body this do { } while
> construct looks odd.  Why not:
>
>         while (old && !atomic_try_cmpxchg(&blkg->use_delay, &old, old - 1))
>                 ;

The form was taken from e6790e4b5d5e97dc287f3496dd2cf2dbabdfdb35 [1].
Using try_cmpxhchg, almost every use fits in

do {
    // the body of the loop
} while (try_cmpxchg ...)

and when the body of the loop is empty, it is clear that this was
indeed intended. Using

while (try_cmpxchg ...);

looks to me like a semicolon was left there in error, like "if (...);".

> The the use of the atomic on ->use_delay looks really whacky to start
> with.  To me it sounds like it really wants to use a proper lock
> instead of all this magic.

I took a lot of care not to change the functionality of the
surrounding code, and any functional change should be outside of the
scope of the patch.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e6790e4b5d5e97dc287f3496dd2cf2dbabdfdb35

Uros.