linux-kernel - Re: Bash not reacting to Ctrl-C

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Date:	Sat, 5 Feb 2011 21:34:22 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Tejun Heo <tj@...nel.org>, roland@...hat.com,
	jan.kratochvil@...hat.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: Bash not reacting to Ctrl-C

On 01/28, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@...hat.com> wrote:
>
> > On 01/28, Ingo Molnar wrote:
> > >
> > > The bug is that occasionally Ctrl-C does not get processed, and that the Ctrl-C is
> > > 'lost'. It can be reproduced here by running ./test-signal several times, and
> > > Ctrl-C-ing it:
> > >
> > >  $ ./test-signal
> > >  ^C
> > >  $ ./test-signal
> > >  ^C^C
> > >  $ ./test-signal
> > >  ^C
> > >
> > > See that '^C^C' line? That is where i had to do Ctrl-C twice.
> >
> > Reproduced.
> >
> > At first glance, /bin/sh should be blamed... Hmm, probably yes,
> > I even reproduced this under strace, and this is what I see
> >
> > 	wait4(-1, 0x7fff388431c4, 0, NULL) = ? ERESTARTSYS (To be restarted)
> > 	--- SIGINT (Interrupt) @ 0 (0) ---
> > 	rt_sigreturn(0)                         = -1 EINTR (Interrupted system call)
> > 	wait4(-1, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, NULL) = 9706
> >
> > So, ^C is not lost, but ./test-signal doesn't want to exit.
>
> Might be some Bash assumption or race that works under other OSs but somehow Linux
> does differently. IIRC Bash is being developed on MacOS-X.
>
> But it's happening all the time (with yum for example - but also with makejobs, as
> Thomas has reported it) - this is simply the first time i managed to reproduce it
> with something really simple.

OK, I seem to understand what happens. Of course I am not sure, I never
looked into these sources before...

Suppose that jctl ^C races with the normal child exit. In this case
waitchld() sets child->status = status (zero in this case) and calls
set_job_status_and_cleanup().

set_job_status_and_cleanup() notice wait_sigint_received and send
SIGINT to itself (termsig_handler (SIGINT)), but somehow it assumes
that the last foreground job should be terminated by SIGINT too:

	 else if (wait_sigint_received && (WTERMSIG (child->status) == SIGINT) &&

Then the next wait_for() clears wait_sigint_received and bash
looses ^C

Oleg.

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