[PATCH] Remove down_write() from taskstats code invoked on the exit() path (bb129994) · Commits · 戴 / test

In send_cpu_listeners(), which is called on the exit path, a down_write() was protecting operations like skb_clone() and genlmsg_unicast() that do GFP_KERNEL allocations. If the oom-killer decides to kill tasks to satisfy the allocations,the exit of those tasks could block on the same semphore. The down_write() was only needed to allow removal of invalid listeners from the listener list. The patch converts the down_write to a down_read and defers the removal to a separate critical region. This ensures that even if the oom-killer is called, no other task's exit is blocked as it can still acquire another down_read. Thanks to Andrew Morton & Herbert Xu for pointing out the oom related pitfalls, and to Chandra Seetharaman for suggesting this fix instead of using something more complex like RCU. Signed-off-by:

Chandra Seetharaman <sekharan@us.ibm.com> Signed-off-by:

Shailabh Nagar <nagar@watson.ibm.com> Signed-off-by:

Andrew Morton <akpm@osdl.org> Signed-off-by:

Linus Torvalds <torvalds@osdl.org>

kernel/taskstats.c

+19 −5

Original line number	Diff line number	Diff line
		@@ -51,6 +51,7 @@ __read_mostly = {
		struct listener {
		struct list_head list;
		pid_t pid;
		char valid;
		};

		struct listener_list {
		@@ -127,7 +128,7 @@ static int send_cpu_listeners(struct sk_buff *skb, unsigned int cpu)
		struct listener s, tmp;
		struct sk_buff skb_next, skb_cur = skb;
		void *reply = genlmsg_data(genlhdr);
		int rc, ret;
		int rc, ret, delcount = 0;

		rc = genlmsg_end(skb, reply);
		if (rc < 0) {
		@@ -137,7 +138,7 @@ static int send_cpu_listeners(struct sk_buff *skb, unsigned int cpu)

		rc = 0;
		listeners = &per_cpu(listener_array, cpu);
		down_write(&listeners->sem);
		down_read(&listeners->sem);
		list_for_each_entry_safe(s, tmp, &listeners->list, list) {
		skb_next = NULL;
		if (!list_is_last(&s->list, &listeners->list)) {
		@@ -150,14 +151,26 @@ static int send_cpu_listeners(struct sk_buff *skb, unsigned int cpu)
		}
		ret = genlmsg_unicast(skb_cur, s->pid);
		if (ret == -ECONNREFUSED) {
		list_del(&s->list);
		kfree(s);
		s->valid = 0;
		delcount++;
		rc = ret;
		}
		skb_cur = skb_next;
		}
		up_write(&listeners->sem);
		up_read(&listeners->sem);

		if (!delcount)
		return rc;

		/* Delete invalidated entries */
		down_write(&listeners->sem);
		list_for_each_entry_safe(s, tmp, &listeners->list, list) {
		if (!s->valid) {
		list_del(&s->list);
		kfree(s);
		}
		}
		up_write(&listeners->sem);
		return rc;
		}

		@@ -290,6 +303,7 @@ static int add_del_listener(pid_t pid, cpumask_t *maskp, int isadd)
		goto cleanup;
		s->pid = pid;
		INIT_LIST_HEAD(&s->list);
		s->valid = 1;

		listeners = &per_cpu(listener_array, cpu);
		down_write(&listeners->sem);

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