linux-kernel - [PATCH] Only send pdeath_signal when getppid changes.

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Message-ID: <m1y7l0xns0.fsf_-_@ebiederm.dsl.xmission.com>
Date:	Tue, 10 Apr 2007 09:16:47 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Robin Holt <holt@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Chris Snook <csnook@...hat.com>, linux-kernel@...r.kernel.org,
	Jack Steiner <steiner@...ricas.sgi.com>,
	Oleg Nesterov <oleg@...sign.ru>,
	Roland McGrath <roland@...hat.com>
Subject: [PATCH] Only send pdeath_signal when getppid changes.

Currently each thread can requrest to be notified when it's parent
terminates, and receive a thread specific signal when that occurs.
That we set this on a per thread granularity and then send it to the
whole thread group seems silly, but whatever.

Currently we send a signal when the results of getppid don't change if
our parent thread dies and we are reparented to a different thread.
This seems counterintuitive to what I would expect when programming in
user space and is a major maitenance issue to continue doing.

The only users I recall using this have non threaded parents and
pdeath_signal predates CLONE_THREAD so arguably this code has been
broken with threaded parents since the day CLONE_THREAD was introduced
and no one ever screamed loudly enough to get it fixed.

So this patch fixes the pdeath_signal behaviour only sending a signal
when the results of getppid would change.

Signed-off-by: Eric W. Biederman <ebiederm@...ssion.com>
---

This patch is against 2.6.21-rc6-mm1 (with utrace applied)
but except for context in the diff that should not matter.

 kernel/exit.c |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 1d91de8..1ec0d1f 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -618,10 +618,6 @@ choose_new_parent(struct task_struct *p, struct task_struct *reaper)
 static void
 reparent_thread(struct task_struct *p, struct task_struct *father)
 {
-	if (p->pdeath_signal)
-		/* We already hold the tasklist_lock here.  */
-		group_send_sig_info(p->pdeath_signal, SEND_SIG_NOINFO, p);
-
 	/* Move the child from its dying parent to the new one.  */
 	list_move_tail(&p->sibling, &p->parent->children);

@@ -635,6 +631,10 @@ reparent_thread(struct task_struct *p, struct task_struct *father)
 	if (p->exit_signal != -1)
 		p->exit_signal = SIGCHLD;

+	if (p->pdeath_signal)
+		/* We already hold the tasklist_lock here.  */
+		group_send_sig_info(p->pdeath_signal, SEND_SIG_NOINFO, p);
+
 	/* If we'd notified the old parent about this child's death,
 	 * also notify the new parent.
 	 */
-- 
1.5.0.g53756

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