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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: George Spelvin <linux@horizon.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Update of file offset on write() etc. is non-atomic with I/O
Date: Mon, 3 Mar 2014 14:55:25 -0800	[thread overview]
Message-ID: <CA+55aFxszctXek3QCXPeDScdSQZvuzp+EXHdA5cZnG3CLeJ1XA@mail.gmail.com> (raw)
In-Reply-To: <CA+55aFwdEbMMTPyCEKop0Fukhxi90OCdvDieX7iB-dX9Ziovdg@mail.gmail.com>

On Mon, Mar 3, 2014 at 1:52 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>>
>> Most of the cases have it kept separately in registers, actually - there's
>> a reason why fdget() and friends are inlined.
>
> Yes. And bit test and set ops on registers are actually cheaper than
> playing around with bytes.

Ugh. gcc gets this *horribly* wrong when it's a bitfield in a union.

The

    f.need_pos_unlock = 1;

thing *should* be just a simple "orl $2,reg", but bitfields seem to
generate really crappy code, and it actually does some insane shifting
and masking crud with the constant "1".

gcc has had problems with bitfields before.

I think I'll respin this with the compat readv/writev case fixed and
with the bitfield replaced with an "unsigned int" that we just do
bitops by hand on. Because that code generation makes me feel slightly
ill.

              Linus

  parent reply	other threads:[~2014-03-03 22:55 UTC|newest]

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-03-03 21:03 Update of file offset on write() etc. is non-atomic with I/O George Spelvin
2014-03-03 21:26 ` Al Viro
2014-03-03 21:52   ` Linus Torvalds
2014-03-03 22:01     ` Al Viro
2014-03-03 22:17       ` Linus Torvalds
2014-03-03 23:28         ` Al Viro
2014-03-03 23:34           ` Linus Torvalds
2014-03-03 23:42             ` Al Viro
2014-03-03 23:59               ` Linus Torvalds
2014-03-04  0:23                 ` Al Viro
2014-03-04  0:42                   ` Linus Torvalds
2014-03-04  1:05                     ` Al Viro
2014-03-04 20:00                       ` Al Viro
2014-03-04 21:17                         ` Linus Torvalds
2014-03-05  0:04                           ` Al Viro
2014-03-10 15:55                             ` Al Viro
2014-03-03 22:55     ` Linus Torvalds [this message]
2014-03-03 23:23       ` Linus Torvalds
2014-03-03 23:39         ` Al Viro
2014-03-03 23:54           ` Linus Torvalds
2014-03-03 23:54           ` Al Viro
2014-03-04 20:11           ` Cedric Blancher
2014-03-04  0:07     ` George Spelvin
2014-05-04  7:04 ` Michael Kerrisk
     [not found] <a8df285f-de7f-4a3a-9a19-e0ad07ab3a5c@blur>
2014-02-20 18:15 ` Zuckerman, Boris
2014-02-20 18:29   ` Al Viro
2014-02-21  6:01     ` Michael Kerrisk (man-pages)
2014-02-23  1:18       ` Kevin Easton
2014-02-23  7:38         ` Michael Kerrisk (man-pages)
  -- strict thread matches above, loose matches on Subject: below --
2014-02-17 15:41 Michael Kerrisk (man-pages)
2014-02-18 13:00 ` Michael Kerrisk
2014-02-20 17:14 ` Linus Torvalds
2014-03-03 17:36   ` Linus Torvalds
2014-03-03 21:45     ` Al Viro
2014-03-03 21:56       ` Linus Torvalds
2014-03-03 22:09         ` Al Viro
2014-03-03 22:20           ` Linus Torvalds
2014-03-03 22:01       ` Linus Torvalds
2014-03-03 22:10         ` Al Viro
2014-03-03 22:22           ` Linus Torvalds
2014-03-06 15:03     ` Michael Kerrisk (man-pages)
2014-03-07  3:38       ` Yongzhi Pan

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