Date: Fri, 27 Jul 2012 11:10:02 GMT From: Bruce Evans <brde@optusnet.com.au> To: freebsd-bugs@FreeBSD.org Subject: Re: kern/170203: [kern] piped dd's don't behave sanely when dealing with a fifo Message-ID: <201207271110.q6RBA2Kj030842@freefall.freebsd.org>
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The following reply was made to PR kern/170203; it has been noted by GNATS.
From: Bruce Evans <brde@optusnet.com.au>
To: davidxu@FreeBSD.org
Cc: Bruce Evans <brde@optusnet.com.au>, 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 21:08:13 +1000 (EST)
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.
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.
Bruce
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