drivers:lmk: Fix null pointer issue

On some race, the tsk that lmk is using may be deleted from the RB tree
by other thread, and rb_next would return a NULL if we use this tsk to
get next. For this case, we need to skip this round of shrink and wait
for the next turn. Otherwise, tsk would trigger NULL pointer panic.

Signed-off-by: Hong-Mei Li <[email protected]>
Reviewed-on: http://gerrit.mot.com/729556
SLTApproved: Slta Waiver <[email protected]>
SME-Granted: SME Approvals Granted
Tested-by: Jira Key <[email protected]>
Reviewed-by: Yi-Wei Zhao <[email protected]>
Submit-Approved: Jira Key <[email protected]>
Signed-off-by: Pranav Vashi <[email protected]>

drivers/staging/android/lowmemorykiller.c

@@ -406,7 +406,7 @@ static int lowmem_shrink(struct shrinker *s, struct shrink_control *sc)
 	rcu_read_lock();
 #ifdef CONFIG_ANDROID_LMK_ADJ_RBTREE
 	for (tsk = pick_first_task();
-		tsk != pick_last_task();
+		tsk != pick_last_task() && tsk != NULL;
 		tsk = pick_next_from_adj_tree(tsk)) {
 #else
 	for_each_process(tsk) {