[PATCH 3/3] staging: android: lowmemorykiller: Drop tasklist_lock usage

[PATCH 3/3] staging: android: lowmemorykiller: Drop tasklist_lock usage

[Anton Vorontsov]

Instead of walking through the whole task list (where we must hold the
task list lock for the whole period while we search for the victim),
let's take procfs' approach of walking up the tgids (for_each_tgit()
walks a pidmap, which internally grabs an rcu lock, but only while
it searches for the next tgid).

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 drivers/staging/android/lowmemorykiller.c |   12 ++++++++----
 1 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/staging/android/lowmemorykiller.c b/drivers/staging/android/lowmemorykiller.c
index 2d8d2b7..a9edb53 100644
--- 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/pid.h>
 #include <linux/profile.h>
 #include <linux/notifier.h>
 
@@ -81,7 +82,8 @@ task_notify_func(struct notifier_block *self, unsigned long val, void *data)
 
 static int lowmem_shrink(struct shrinker *s, struct shrink_control *sc)
 {
-	struct task_struct *p;
+	struct pid_namespace *ns = &init_pid_ns;
+	struct tgid_iter iter = {};
 	struct task_struct *selected = NULL;
 	int rem = 0;
 	int tasksize;
@@ -132,8 +134,8 @@ static int lowmem_shrink(struct shrinker *s, struct shrink_control *sc)
 	}
 	selected_oom_adj = min_adj;
 
-	read_lock(&tasklist_lock);
-	for_each_process(p) {
+	for_each_tgid(ns, iter) {
+		struct task_struct *p = iter.task;
 		struct mm_struct *mm;
 		struct signal_struct *sig;
 		int oom_adj;
@@ -160,7 +162,9 @@ static int lowmem_shrink(struct shrinker *s, struct shrink_control *sc)
 			if (oom_adj == selected_oom_adj &&
 			    tasksize <= selected_tasksize)
 				continue;
+			put_task_struct(selected);
 		}
+		get_task_struct(p);
 		selected = p;
 		selected_tasksize = tasksize;
 		selected_oom_adj = oom_adj;
@@ -181,11 +185,11 @@ static int lowmem_shrink(struct shrinker *s, struct shrink_control *sc)
 		task_handoff_register(&task_nb);
 #endif
 		force_sig(SIGKILL, selected);
+		put_task_struct(selected);
 		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);
 	return rem;
 }
 
-- 
1.7.7

Re: [PATCH 3/3] staging: android: lowmemorykiller: Drop tasklist_lock usage

[Oleg Nesterov]

On 01/30, Anton Vorontsov wrote:
>
> @@ -132,8 +134,8 @@ static int lowmem_shrink(struct shrinker *s, struct shrink_control *sc)
>  	}
>  	selected_oom_adj = min_adj;
>
> -	read_lock(&tasklist_lock);
> -	for_each_process(p) {
> +	for_each_tgid(ns, iter) {

This makes force_sig() below unsafe, it can race with the exiting task.

Oleg.