[PATCH for mmotm] oom: remove unnecessary task->sighand->siglock grabbing.

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

Subject: [PATCH] oom: remove unnecessary task->sighand->siglock grabbing.

Oleg Nesterov get_oom_adj() doesn't need grab task->sighand->siglock.
tasklist_lock already prevent any race.

This patch remove its unnecessary lock.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
 mm/oom_kill.c |   29 +++++++----------------------
 1 file changed, 7 insertions(+), 22 deletions(-)

Index: b/mm/oom_kill.c
===================================================================
--- a/mm/oom_kill.c	2009-08-19 00:25:28.000000000 +0900
+++ b/mm/oom_kill.c	2009-08-19 00:43:31.000000000 +0900
@@ -34,17 +34,13 @@ int sysctl_oom_dump_tasks;
 static DEFINE_SPINLOCK(zone_scan_lock);
 /* #define DEBUG */
 
-int get_oom_adj(struct task_struct *tsk)
+static int get_oom_adj(struct task_struct *tsk)
 {
-	unsigned long flags;
-	int oom_adj = OOM_DISABLE;
-
-	if (tsk->mm && lock_task_sighand(tsk, &flags)) {
-		oom_adj = tsk->signal->oom_adj;
-		unlock_task_sighand(tsk, &flags);
-	}
+	/* OOM Killer can't kill kernel thread */
+	if (!tsk->mm)
+		return OOM_DISABLE;
 
-	return oom_adj;
+	return tsk->signal->oom_adj;
 }
 
 void set_oom_adj(struct task_struct *tsk, int oom_adj)
@@ -97,7 +93,6 @@ unsigned long badness(struct task_struct
 	struct task_struct *child;
 	int oom_adj;
 	struct task_cputime task_time;
-	unsigned long flags;
 	unsigned long utime;
 	unsigned long stime;
 
@@ -105,10 +100,7 @@ unsigned long badness(struct task_struct
 	if (oom_adj == OOM_DISABLE)
 		return 0;
 
-	if (!lock_task_sighand(p, &flags))
-		return 0;
 	thread_group_cputime(p, &task_time);
-	unlock_task_sighand(p, &flags);
 
 	task_lock(p);
 	mm = p->mm;
@@ -329,8 +321,6 @@ static struct task_struct *select_bad_pr
 static void dump_tasks(const struct mem_cgroup *mem)
 {
 	struct task_struct *g, *p;
-	unsigned long flags;
-	int oom_adj;
 
 	printk(KERN_INFO "[ pid ]   uid  tgid total_vm      rss cpu oom_adj "
 	       "name\n");
@@ -342,12 +332,6 @@ static void dump_tasks(const struct mem_
 		if (!thread_group_leader(p))
 			continue;
 
-		if (!lock_task_sighand(p, &flags))
-			continue;
-
-		oom_adj = p->signal->oom_adj;
-		unlock_task_sighand(p, &flags);
-
 		task_lock(p);
 		mm = p->mm;
 		if (!mm) {
@@ -361,7 +345,8 @@ static void dump_tasks(const struct mem_
 		}
 		printk(KERN_INFO "[%5d] %5d %5d %8lu %8lu %3d     %3d %s\n",
 		       p->pid, __task_cred(p)->uid, p->tgid, mm->total_vm,
-		       get_mm_rss(mm), (int)task_cpu(p), oom_adj, p->comm);
+		       get_mm_rss(mm), (int)task_cpu(p), get_oom_adj(p),
+		       p->comm);
 		task_unlock(p);
 	} while_each_thread(g, p);
 }


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