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>

diff --git a/kernel/exit.c b/kernel/exit.c
index 171c9a9d7b006479c1594780152ea38c6085d0a3..decf648574f6c935f8266af0e40d9e8d3e6ff846 100644 (file)
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -802,13 +802,13 @@ 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
         */
        flush_ptrace_hw_breakpoint(tsk);
 
        exit_notify(tsk, group_dead);
+       proc_exit_connector(tsk);
 #ifdef CONFIG_NUMA
        task_lock(tsk);
        mpol_put(tsk->mempolicy);