linux-kernel - Re: Update of file offset on write() etc. is non-atomic with I/O

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Date:	Mon, 3 Mar 2014 14:55:25 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	George Spelvin <linux@...izon.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Update of file offset on write() etc. is non-atomic with I/O

On Mon, Mar 3, 2014 at 1:52 PM, Linus Torvalds
<torvalds@...ux-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
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