linux-kernel - Re: Hang opening a pipe written to by a child, with SIGCHLD

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Date:	Mon, 02 Apr 2012 10:49:10 -0400
From:	Shea Levy <shea@...alevy.com>
To:	Lluís Batlle i Rossell <viric@...ic.name>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Hang opening a pipe written to by a child, with SIGCHLD

On 04/02/2012 10:46 AM, Lluís Batlle i Rossell wrote:
> Hello,
>
> I had troubles running a simple bash script where a child wrote to a named pipe
> before dying, and the parent opened and read the pipe.
>
> On a computer, my script hangs always. Also under 'strace'. Not under 'strace
> -f'. Thinking of some race, I wrote a small script that hangs in the linuxes I
> could try (3.2.11 now):
>
> ----------
> #!/var/run/current-system/sw/bin/bash
>
> PIPE=/tmp/pipe
>
> rm -f $PIPE
> mkfifo $PIPE
>
> function spawn {
>      echo DONE>  $PIPE
> }
>
> spawn sleep 1&
>
> while true; do
>      echo reading
>      while read LINE<  $PIPE; do
>          echo $LINE
>          spawn&
>      done
> done
> -------------
>
> bash uses SA_RESTART on SIGCHLD handler, and nevertheless the script halts at
> printing some amount of 'DONE'. Strace shows it's inside the open() call, while the open
> call has been interrupted by SIGCHLD. One of the computers I have hangs at the *first* DONE always, and that's why it's annoying to me.
>
> Can anyone reproduce this?

I've reproduced this on my system as well (3.3). No idea about cause though.

~Shea
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