From: Guillaume Morin <guillaume@morinfr.org>
Date: Mon, 7 Apr 2014 22:38:31 +0000 (-0700)
Subject: kernel/exit.c: call proc_exit_connector() after exit_state is set
X-Git-Tag: v3.15-rc1~57^2~72
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=ef9823939e5acd5d323ff61fbc427ef998dd203e;p=pandora-kernel.git

kernel/exit.c: call proc_exit_connector() after exit_state is set

The process events connector delivers a notification when a process
exits.  This is really convenient for a process that spawns and wants to
monitor its children through an epoll-able() interface.

Unfortunately, there is a small window between when the event is
delivered and the child become wait()-able.

This is creates a race if the parent wants to make sure that it knows
about the exit, e.g

pid_t pid = fork();
if (pid > 0) {
	register_interest_for_pid(pid);
	if (waitpid(pid, NULL, WNOHANG) > 0)
	{
	  /* We might have raced with exit() */
	}
	return;
}

/* Child */
execve(...)

register_interest_for_pid() would be telling the the connector socket
reader to pay attention to events related to pid.

Though this is not a bug, I think it would make the connector a bit more
usable if this race was closed by simply moving the call to
proc_exit_connector() from just before exit_notify() to right after.

Oleg said:

: Even with this patch the code above is still "racy" if the child is
: multi-threaded.  Plus it should obviously filter-out subthreads.  And
: afaics there is no way to make it reliable, even if you change the code
: above so that waitpid() is called only after the last thread exits WNOHANG
: still can fail.

Signed-off-by: Guillaume Morin <guillaume@morinfr.org>
Cc: Matt Helsley <matt.helsley@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---

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