Date: Fri, 27 Jul 2012 12:30:10 GMT From: David Xu <listlog2011@gmail.com> To: freebsd-bugs@FreeBSD.org Subject: Re: kern/170203: [kern] piped dd's don't behave sanely when dealing with a fifo Message-ID: <201207271230.q6RCUAu8043530@freefall.freebsd.org>
next in thread | raw e-mail | index | archive | help
The following reply was made to PR kern/170203; it has been noted by GNATS.
From: David Xu <listlog2011@gmail.com>
To: Bruce Evans <brde@optusnet.com.au>
Cc: davidxu@FreeBSD.org, Garrett Cooper <yanegomi@gmail.com>,
freebsd-bugs@FreeBSD.org, freebsd-gnats-submit@FreeBSD.org
Subject: Re: kern/170203: [kern] piped dd's don't behave sanely when dealing
with a fifo
Date: Fri, 27 Jul 2012 20:29:02 +0800
On 2012/7/27 19:08, Bruce Evans wrote:
> On Fri, 27 Jul 2012, David Xu wrote:
>
>> On 2012/7/27 10:07, Bruce Evans wrote:
>>>
>>> I think it's working almost as expected. Large blocks give non-atomic
>>> I/O, so the reader sees small blocks, then EOF when it gets ahead of
>>> the writer. This always happens without SMP.
>>>
>>> Not is a bug (debugged below). There is no SIGPIPE at the start of
>>> write() because there is a reader then, and no SIGPIPE for the next
>>> write() because there is no next write() -- the current one doesn't
>>> notice when the reader goes away.
>>>
>> After fixed dd to not open fifo output file in O_RDWR mode, I still
>> found the
>> writer is blocked there even the reader is already exited.
>
> I'm not sure that dd's open is a bug. It must be intentional to use
> O_RDWR for some cases.
>
Don't know if original author even thought about FIFO.
> POSIX (old 2001 draft) doesn't say anything about dd's open mode.
>
>> I think this is definitely a bug. if reader is exited, the writer
>> should be aborted too,
>> but I found it still be blocked in state "pipedwt", obviously, the
>> code in
>> /sys/fs/fifo_vnops.c wants to wake up the writer when the reader is
>> closing the fifo,
>> but it failed, because the bit flag PIPE_WANTW is forgotten to be set
>> by writer,
>> so it skips executing wakeup(), and then the writer has no chance to
>> find EOF bit flag
>> is set.
>
> Does this affect nameless pipes too? The old implementation presumably
> doesn't have this bug.
>
It is easy to repeat the bug for named pipes, don't know if nameless
pipes have
same bug, I can not reproduce it yet.
> Bruce
> .
>
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?201207271230.q6RCUAu8043530>