linux-kernel - [PATCH v2] mm, oom: Fix race when selecting process to kill

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Message-Id: <1383934035-933-1-git-send-email-snanda@chromium.org>
Date:	Fri,  8 Nov 2013 10:07:15 -0800
From:	Sameer Nanda <snanda@...omium.org>
To:	akpm@...ux-foundation.org, mhocko@...e.cz, rientjes@...gle.com,
	hannes@...xchg.org, rusty@...tcorp.com.au, semenzato@...gle.com,
	murzin.v@...il.com, oleg@...hat.com, dserrg@...il.com,
	msb@...omium.org
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Sameer Nanda <snanda@...omium.org>
Subject: [PATCH v2] mm, oom: Fix race when selecting process to kill

The selection of the process to be killed happens in two spots:
first in select_bad_process and then a further refinement by
looking for child processes in oom_kill_process. Since this is
a two step process, it is possible that the process selected by
select_bad_process may get a SIGKILL just before oom_kill_process
executes. If this were to happen, __unhash_process deletes this
process from the thread_group list. This results in oom_kill_process
getting stuck in an infinite loop when traversing the thread_group
list of the selected process.

Fix this race by adding a pid_alive check for the selected process
with tasklist_lock held in oom_kill_process.

Change-Id: I865c64486ccfc0e4818e7045a8fa3353e2fb63f8
Signed-off-by: Sameer Nanda <snanda@...omium.org>
---
 mm/oom_kill.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 6738c47..af99b1a 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -412,13 +412,16 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 	static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
 					      DEFAULT_RATELIMIT_BURST);
 
+	read_lock(&tasklist_lock);
+
 	/*
 	 * If the task is already exiting, don't alarm the sysadmin or kill
 	 * its children or threads, just set TIF_MEMDIE so it can die quickly
 	 */
-	if (p->flags & PF_EXITING) {
+	if (p->flags & PF_EXITING || !pid_alive(p)) {
 		set_tsk_thread_flag(p, TIF_MEMDIE);
 		put_task_struct(p);
+		read_unlock(&tasklist_lock);
 		return;
 	}
 
@@ -436,7 +439,6 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 	 * parent.  This attempts to lose the minimal amount of work done while
 	 * still freeing memory.
 	 */
-	read_lock(&tasklist_lock);
 	do {
 		list_for_each_entry(child, &t->children, sibling) {
 			unsigned int child_points;
-- 
1.8.4.1

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