linux-kernel - [PATCH] exit.c: call proc_exit_connector() after exit

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Date:	Mon, 24 Feb 2014 20:27:34 +0100
From:	Guillaume Morin <guillaume@...infr.org>
To:	matt.helsley@...il.com, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH] exit.c: call proc_exit_connector() after exit_state is set

From: Guillaume Morin <guillaume@...infr.org>

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.

Signed-off-by: Guillaume Morin <guillaume@...infr.org>
Cc: matt.helsley@...il.com
---
diff --git a/kernel/exit.c b/kernel/exit.c
index 1e77fc6..9b0ac8c 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -804,7 +804,6 @@ void do_exit(long code)
 
 	module_put(task_thread_info(tsk)->exec_domain->module);
 
-	proc_exit_connector(tsk);
 
 	/*
 	 * FIXME: do that only when needed, using sched_exit tracepoint
@@ -812,6 +811,7 @@ void do_exit(long code)
 	flush_ptrace_hw_breakpoint(tsk);
 
 	exit_notify(tsk, group_dead);
+	proc_exit_connector(tsk);
 #ifdef CONFIG_NUMA
 	task_lock(tsk);
 	mpol_put(tsk->mempolicy);



-- 
Guillaume Morin <guillaume@...infr.org>
--
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