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Subject[PATCH v2] mm, oom: Fix race when selecting process to kill
Date
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@chromium.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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