mm: optimize the redundant loop of mm_update_owner_next()

When mm_update_owner_next() 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.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Jinliang Zheng <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Mateusz Guzik <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Tycho Andersen <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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) {