public inbox for [email protected]
 help / color / mirror / Atom feed
From: Peter Zijlstra <[email protected]>
To: Will Deacon <[email protected]>
Cc: "Jens Axboe" <[email protected]>,
	"Carter Li 李通洲" <[email protected]>,
	"Pavel Begunkov" <[email protected]>,
	io-uring <[email protected]>,
	"Oleg Nesterov" <[email protected]>,
	"Mark Rutland" <[email protected]>,
	[email protected]
Subject: Re: [PATCH] asm-generic/atomic: Add try_cmpxchg() fallbacks
Date: Thu, 20 Feb 2020 11:37:27 +0100	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <20200220103044.GA13608@willie-the-truck>

On Thu, Feb 20, 2020 at 10:30:45AM +0000, Will Deacon wrote:
> On Tue, Feb 18, 2020 at 03:27:00PM +0100, Peter Zijlstra wrote:
> > On Tue, Feb 18, 2020 at 02:13:10PM +0100, Peter Zijlstra wrote:
> > > (with the caveat that try_cmpxchg() doesn't seem available on !x86 -- I
> > > should go fix that)
> > 
> > Completely untested (lemme go do that shortly), but something like so I
> > suppose.
> > 
> > ---
> > Subject: asm-generic/atomic: Add try_cmpxchg() fallbacks
> > 
> > Only x86 provides try_cmpxchg() outside of the atomic_t interfaces,
> > provide generic fallbacks to create this interface from the widely
> > available cmpxchg() function.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
> > ---
> > diff --git a/include/linux/atomic-fallback.h b/include/linux/atomic-fallback.h
> > index 656b5489b673..243f61d6c35f 100644
> > --- a/include/linux/atomic-fallback.h
> > +++ b/include/linux/atomic-fallback.h
> > @@ -77,6 +77,50 @@
> >  
> >  #endif /* cmpxchg64_relaxed */
> >  
> > +#ifndef try_cmpxchg
> > +#define try_cmpxchg(_ptr, _oldp, _new) \
> > +({ \
> > +	typeof(*ptr) ___r, ___o = *(_oldp); \
> 
> Probably worth pointing out that this will have the nasty behaviour
> for volatile pointers that we're tackling for READ_ONCE. Obviously no
> need to hold this up, but just mentioning it here in the hope that one
> of us remembers to fix it later on.

Right :/

> > diff --git a/scripts/atomic/gen-atomic-fallback.sh b/scripts/atomic/gen-atomic-fallback.sh
> > index b6c6f5d306a7..3c9be8d550e0 100755
> > --- a/scripts/atomic/gen-atomic-fallback.sh
> > +++ b/scripts/atomic/gen-atomic-fallback.sh
> > @@ -140,6 +140,32 @@ cat <<EOF
> >  EOF
> >  }
> >  
> > +gen_try_cmpxchg_fallback()
> > +{
> > +	local order="$1"; shift;
> > +
> > +cat <<EOF
> > +#ifndef try_cmpxchg${order}
> > +#define try_cmpxchg${order}(_ptr, _oldp, _new) \\
> > +({ \\
> > +	typeof(*ptr) ___r, ___o = *(_oldp); \\
> > +	___r = cmpxchg${order}((_ptr), ___o, (_new)); \\
> > +	if (unlikely(___r != ___o)) \\
> > +		*(_old) = ___r; \\
> 
> This doesn't compile because _old isn't declared. Probably best to avoid
> evaluating _oldp twice though.

Compiler had already spotted that, I'll make it something like:

	typeof(*ptr) *___op = (_oldp), ___o = *___op;

	...

		*___op = ___r;

Which avoids the double eval.

  reply	other threads:[~2020-02-20 10:37 UTC|newest]

Thread overview: 59+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-12 16:31 [ISSUE] The time cost of IOSQE_IO_LINK Carter Li 李通洲
2020-02-12 17:11 ` Jens Axboe
2020-02-12 17:22   ` Jens Axboe
2020-02-12 17:29     ` Jens Axboe
2020-02-13  0:33   ` Carter Li 李通洲
2020-02-13 15:08     ` Pavel Begunkov
2020-02-13 15:14       ` Jens Axboe
2020-02-13 15:51         ` Carter Li 李通洲
2020-02-14  1:25           ` Carter Li 李通洲
2020-02-14  2:45             ` Jens Axboe
2020-02-14  5:03               ` Jens Axboe
2020-02-14 15:32                 ` Peter Zijlstra
2020-02-14 15:47                   ` Jens Axboe
2020-02-14 16:18                     ` Jens Axboe
2020-02-14 17:52                       ` Jens Axboe
2020-02-14 20:44                         ` Jens Axboe
2020-02-15  0:16                           ` Carter Li 李通洲
2020-02-15  1:10                             ` Jens Axboe
2020-02-15  1:25                               ` Carter Li 李通洲
2020-02-15  1:27                                 ` Jens Axboe
2020-02-15  6:01                                   ` Jens Axboe
2020-02-15  6:32                                     ` Carter Li 李通洲
2020-02-15 15:11                                       ` Jens Axboe
2020-02-16 19:06                                     ` Pavel Begunkov
2020-02-16 22:23                                       ` Jens Axboe
2020-02-17 10:30                                         ` Pavel Begunkov
2020-02-17 19:30                                           ` Jens Axboe
2020-02-16 23:06                                       ` Jens Axboe
2020-02-16 23:07                                         ` Jens Axboe
2020-02-17 12:09                           ` Peter Zijlstra
2020-02-17 16:12                             ` Jens Axboe
2020-02-17 17:16                               ` Jens Axboe
2020-02-17 17:46                                 ` Peter Zijlstra
2020-02-17 18:16                                   ` Jens Axboe
2020-02-18 13:13                                     ` Peter Zijlstra
2020-02-18 14:27                                       ` [PATCH] asm-generic/atomic: Add try_cmpxchg() fallbacks Peter Zijlstra
2020-02-18 14:40                                         ` Peter Zijlstra
2020-02-20 10:30                                         ` Will Deacon
2020-02-20 10:37                                           ` Peter Zijlstra [this message]
2020-02-20 10:39                                             ` Will Deacon
2020-02-18 14:56                                       ` [ISSUE] The time cost of IOSQE_IO_LINK Oleg Nesterov
2020-02-18 15:07                                         ` Oleg Nesterov
2020-02-18 15:38                                           ` Peter Zijlstra
2020-02-18 16:33                                             ` Jens Axboe
2020-02-18 15:07                                         ` Peter Zijlstra
2020-02-18 15:50                                           ` [PATCH] task_work_run: don't take ->pi_lock unconditionally Oleg Nesterov
2020-02-20 16:39                                             ` Peter Zijlstra
2020-02-20 17:22                                               ` Oleg Nesterov
2020-02-20 17:49                                                 ` Peter Zijlstra
2020-02-21 14:52                                                   ` Oleg Nesterov
2020-02-24 18:47                                                     ` Jens Axboe
2020-02-28 19:17                                                       ` Jens Axboe
2020-02-28 19:25                                                         ` Peter Zijlstra
2020-02-28 19:28                                                           ` Jens Axboe
2020-02-28 20:06                                                             ` Peter Zijlstra
2020-02-28 20:15                                                               ` Jens Axboe
2020-02-18 16:46                                       ` [ISSUE] The time cost of IOSQE_IO_LINK Jens Axboe
2020-02-18 16:52                                         ` Jens Axboe
2020-02-18 13:13                               ` Peter Zijlstra

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200220103727.GW18400@hirez.programming.kicks-ass.net \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox