linux-kernel - [PATCH] de_thread: Move notify

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Date:	Thu, 5 Feb 2015 16:13:20 +0300
From:	Kirill Tkhai <ktkhai@...allels.com>
To:	<linux-kernel@...r.kernel.org>
CC:	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>, <tkhai@...dex.ru>
Subject: [PATCH] de_thread: Move notify_count write under lock


The write operation may be reordered with the setting of group_exit_task.
If so, this fires in exit_notify().

Looks like, it's not good to add smp barriers for this case, especially
in exit_notify(), so let's put the notify_count write under write lock.

Signed-off-by: Kirill Tkhai <ktkhai@...allels.com>
---
 fs/exec.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/exec.c b/fs/exec.c
index ad8798e..42782d5 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -920,10 +920,16 @@ static int de_thread(struct task_struct *tsk)
 	if (!thread_group_leader(tsk)) {
 		struct task_struct *leader = tsk->group_leader;
 
-		sig->notify_count = -1;	/* for exit_notify() */
 		for (;;) {
 			threadgroup_change_begin(tsk);
 			write_lock_irq(&tasklist_lock);
+			/*
+			 * We could set it once outside the for() cycle, but
+			 * this requires to use SMP barriers there and in
+			 * exit_notify(), because the write operation may
+			 * be reordered with the setting of group_exit_task.
+			 */
+			sig->notify_count = -1;	/* for exit_notify() */
 			if (likely(leader->exit_state))
 				break;
 			__set_current_state(TASK_KILLABLE);



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