linux-kernel - Re: [PATCHSET] ptrace,signal: group stop / ptrace updates

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Date:	Fri, 28 Jan 2011 18:55:33 +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: [PATCHSET] ptrace,signal: group stop / ptrace updates

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.




This is what ./test-signal does when ^C does work:

	wait4(-1, 0x7fff1c283b74, 0, NULL)      = ? ERESTARTSYS (To be restarted)
	--- SIGINT (Interrupt) @ 0 (0) ---
	rt_sigreturn(0)                         = -1 EINTR (Interrupted system call)

OK, it doesn't exit immediately, but then it kills itself:

	wait4(-1, [{WIFSIGNALED(s) && WTERMSIG(s) == SIGINT}], 0, NULL) = 19585
	rt_sigaction(SIGINT, {SIG_DFL, [], SA_RESTORER, 0x7f3c3035b150}, {0x433d30, [], SA_RESTORER, 0x7f3c3035b150}, 8) = 0
	rt_sigaction(SIGINT, {SIG_DFL, [], SA_RESTORER, 0x7f3c3035b150}, {SIG_DFL, [], SA_RESTORER, 0x7f3c3035b150}, 8) = 0
	kill(19584, SIGINT)




Looking into the previous log (when it doesn't exit) again,

	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
	rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
	--- SIGCHLD (Child exited) @ 0 (0) ---
	wait4(-1, 0x7fff38842d24, WNOHANG, NULL) = -1 ECHILD (No child processes)
	rt_sigreturn(0x8)                       = 0
	rt_sigaction(SIGINT, {SIG_DFL, [], SA_RESTORER, 0x7f3cbdbd0150}, {0x433d30, [], SA_RESTORER, 0x7f3cbdbd0150}, 8) = 0
	rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
	rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
	rt_sigprocmask(SIG_BLOCK, [INT CHLD], [], 8) = 0
	clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f3cbe9ab780) = 9707

Perhaps the handler for SIGCHLD clears some internal i_am_going_to_exit flag,
I dunno.

Oleg.

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