staging: android/lowmemorykiller: Don't grab tasklist_lock

Grabbing tasklist_lock has its disadvantages, i.e. it blocks
process creation and destruction. If there are lots of processes,
blocking doesn't sound as a great idea.

For LMK, it is sufficient to surround tasks list traverse with
rcu_read_{,un}lock().

>From now on using force_sig() is not safe, as it can race with an
already exiting task, so we use send_sig() now. As a downside, it
won't kill PID namespace init processes, but that's not what we
want anyway.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/staging/android/lowmemorykiller.c b/drivers/staging/android/lowmemorykiller.c
index 2d8d2b79610190ade925b09250690cea52328fb7..63da8447218f84ba739ca4a941cb867c69a642ca 100644 (file)
--- a/drivers/staging/android/lowmemorykiller.c
+++ b/drivers/staging/android/lowmemorykiller.c
@@ -34,6 +34,7 @@
 #include <linux/mm.h>
 #include <linux/oom.h>
 #include <linux/sched.h>
+#include <linux/rcupdate.h>
 #include <linux/profile.h>
 #include <linux/notifier.h>
 
@@ -132,7 +133,7 @@ static int lowmem_shrink(struct shrinker *s, struct shrink_control *sc)
        }
        selected_oom_adj = min_adj;
 
-       read_lock(&tasklist_lock);
+       rcu_read_lock();
        for_each_process(p) {
                struct mm_struct *mm;
                struct signal_struct *sig;
@@ -180,12 +181,12 @@ static int lowmem_shrink(struct shrinker *s, struct shrink_control *sc)
                lowmem_deathpending = selected;
                task_handoff_register(&task_nb);
 #endif
-               force_sig(SIGKILL, selected);
+               send_sig(SIGKILL, selected, 0);
                rem -= selected_tasksize;
        }
        lowmem_print(4, "lowmem_shrink %lu, %x, return %d\n",
                     sc->nr_to_scan, sc->gfp_mask, rem);
-       read_unlock(&tasklist_lock);
+       rcu_read_unlock();
        return rem;
 }