taskstats: add_del_listener() shouldn't use the wrong node

1. Commit 26c4caea9d69 "don't allow duplicate entries in listener mode"
   changed add_del_listener(REGISTER) so that "next_cpu:" can reuse the
   listener allocated for the previous cpu, this doesn't look exactly
   right even if minor.

   Change the code to kfree() in the already-registered case, this case
   is unlikely anyway so the extra kmalloc_node() shouldn't hurt but
   looke more correct and clean.

2. use the plain list_for_each_entry() instead of _safe() to scan
   listeners->list.

3. Remove the unneeded INIT_LIST_HEAD(&s->list), we are going to
   list_add(&s->list).

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Vasiliy Kulikov <segoon@openwall.com>
Cc: Balbir Singh <bsingharora@gmail.com>
Reviewed-by: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 9 deletions

diff --git a/kernel/taskstats.c b/kernel/taskstats.c
index d1db2880d1cf..a09a54936f19 100644
--- a/kernel/taskstats.c
+++ b/kernel/taskstats.c
@@ -291,30 +291,28 @@ static int add_del_listener(pid_t pid, const struct cpumask *mask, int isadd)
         if (!cpumask_subset(mask, cpu_possible_mask))
                 return -EINVAL;
 
-        s = NULL;
         if (isadd == REGISTER) {
                 for_each_cpu(cpu, mask) {
-                        if (!s)
-                                s = kmalloc_node(sizeof(struct listener),
-                                                 GFP_KERNEL, cpu_to_node(cpu));
+                        s = kmalloc_node(sizeof(struct listener),
+                                        GFP_KERNEL, cpu_to_node(cpu));
                         if (!s)
                                 goto cleanup;
+
                         s->pid = pid;
-                        INIT_LIST_HEAD(&s->list);
                         s->valid = 1;
 
                         listeners = &per_cpu(listener_array, cpu);
                         down_write(&listeners->sem);
-                        list_for_each_entry_safe(s2, tmp, &listeners->list, list) {
+                        list_for_each_entry(s2, &listeners->list, list) {
                                 if (s2->pid == pid)
-                                        goto next_cpu;
+                                        goto exists;
                         }
                         list_add(&s->list, &listeners->list);
                         s = NULL;
-next_cpu:
+exists:
                         up_write(&listeners->sem);
+                        kfree(s); /* nop if NULL */
                 }
-                kfree(s);
                 return 0;
         }