linux-kernel - Re: Possible bug introduced in commit 9b84cca

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Message-ID: <20111229113245.GA18062@redhat.com>
Date:	Thu, 29 Dec 2011 12:32:45 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Tejun Heo <tj@...nel.org>, Denys Vlasenko <dvlasenk@...hat.com>,
	linux-kernel@...r.kernel.org,
	Łukasz Michalik <lmi@....uni.wroc.pl>,
	"Dmitry V. Levin" <ldv@...linux.org>
Subject: Re: Possible bug introduced in commit 9b84cca

On 12/28, Denys Vlasenko wrote:
>
> Looks like after commit 9b84cca, waitpid under strace
> sometimes returns bogus ECHILD while child does exist.
>
> I did not yet confirm that the bug appeared exactly
> at this commit - Łukasz says that.
>
> I confirmed that bug exists on kernels 3.1.6 (in Fedora)
> and 3.1.0-rc4 (vanilla).
>
> We have a testcase which spawns N threads, each of them
> performs an infinite loop "fork, exit in child, waitpid
> in parent for the child". When straced, sometimes waitpid
> returns ECHILD.

You mean, the natural parent gets ECHILD, not strace?

> The key part is here:
>
> 931   clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0xf763dbd8) = 1048
> 1048  exit_group(42)                    = ?
> 931   waitpid(1048,  <unfinished ...>
> 1048  +++ exited with 42 +++
> 931   <... waitpid resumed> 0xf763d3a0, 0) = -1 ECHILD (No child processes)

Argh. I seem to understand

I didn't check, but I think the offending commit is 823b018e5b1196d8
"job control: Small reorganization of wait_consider_task()".

ptracer sees EXIT_ZOMBIE and temporary sets EXIT_DEAD, this fools
the ->real_parent.

I need to think. The fix should be simple, but perhaps it is the
time to kill EXIT_DEAD altogether. I'll try to make the patch
after vacation. In the next year ;)

Thanks a lot Denys!

Oleg.

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