From: "H. Peter Anvin" <hpa@zytor.com>
To: paulmck@linux.ibm.com
Cc: Alexander Potapenko <glider@google.com>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Dmitriy Vyukov <dvyukov@google.com>,
James Y Knight <jyknight@google.com>
Subject: Re: Potentially missing "memory" clobbers in bitops.h for x86
Date: Fri, 29 Mar 2019 14:51:26 -0700 [thread overview]
Message-ID: <8092b8aa-bb1c-0266-b308-5cebfb25e2ef@zytor.com> (raw)
In-Reply-To: <20190329210918.GZ4102@linux.ibm.com>
On 3/29/19 2:09 PM, Paul E. McKenney wrote:
>>
>> Note: the atomic versions of these functions obviously need to have
>> "volatile" and the clobber anyway, as they are by definition barriers
>> and moving memory operations around them would be a very serious error.
>
> The atomic functions that return void don't need to order anything except
> the input and output arguments. The oddness with clear_bit() is that the
> memory changed isn't necessarily the quantity referenced by the argument,
> if the number of bits specified is large.
>
> So (for example) atomic_inc() does not need a "memory" clobber, right?
>
I don't believe that is true: the code calling it has a reasonable
expectation that previous memory operations have finished and later
memory operations have not started from the point of view of another
processor. You are more of an expert on memory ordering than I am, but
I'm 89% sure that there is plenty of code in the kernel which makes that
assumption.
-hpa
next prev parent reply other threads:[~2019-03-29 21:51 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-28 14:14 Potentially missing "memory" clobbers in bitops.h for x86 Alexander Potapenko
2019-03-28 16:22 ` Paul E. McKenney
2019-03-29 15:54 ` Alexander Potapenko
2019-03-29 20:52 ` H. Peter Anvin
2019-03-29 21:09 ` Paul E. McKenney
2019-03-29 21:51 ` H. Peter Anvin [this message]
2019-03-29 22:05 ` Paul E. McKenney
2019-03-29 22:30 ` hpa
2019-04-01 10:53 ` Peter Zijlstra
2019-04-01 15:44 ` Paul E. McKenney
2019-04-01 16:04 ` Peter Zijlstra
2019-04-01 15:00 ` Alexander Potapenko
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=8092b8aa-bb1c-0266-b308-5cebfb25e2ef@zytor.com \
--to=hpa@zytor.com \
--cc=dvyukov@google.com \
--cc=glider@google.com \
--cc=jyknight@google.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@kernel.org \
--cc=paulmck@linux.ibm.com \
--cc=peterz@infradead.org \
/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 an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.