[PATCH v2] mm: optimize the redundant loop of mm_update_next_owner()

[alexjlzheng]

From: Jinliang Zheng <alexjlzheng@xxxxxxxxxxx>

When mm_update_next_owner() is racing with swapoff (try_to_unuse()) or /proc or
ptrace or page migration (get_task_mm()), it is impossible to find an
appropriate task_struct in the loop whose mm_struct is the same as the target
mm_struct.

If the above race condition is combined with the stress-ng-zombie and
stress-ng-dup tests, such a long loop can easily cause a Hard Lockup in
write_lock_irq() for tasklist_lock.

Recognize this situation in advance and exit early.

Signed-off-by: Jinliang Zheng <alexjlzheng@xxxxxxxxxxx>
---
Changelog:

V2: Fix mm_update_owner_next() to mm_update_next_owner() in comment
---
 kernel/exit.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/exit.c b/kernel/exit.c
index f95a2c1338a8..81fcee45d630 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -484,6 +484,8 @@ void mm_update_next_owner(struct mm_struct *mm)
 	 * Search through everything else, we should not get here often.
 	 */
 	for_each_process(g) {
+		if (atomic_read(&mm->mm_users) <= 1)
+			break;
 		if (g->flags & PF_KTHREAD)
 			continue;
 		for_each_thread(g, c) {
-- 
2.39.3