[PATCH] Remove down_write() from taskstats code invoked on the exit() path

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>

1 files changed, 19 insertions, 5 deletions

diff --git a/kernel/taskstats.c b/kernel/taskstats.c
index abb59e323544..f45179ce028e 100644
--- a/kernel/taskstats.c
+++ b/kernel/taskstats.c
@@ -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);