signals: Fix more rcu assumptions

1) Remove the misleading comment in __sigqueue_alloc() which claims
   that holding a spinlock is equivalent to rcu_read_lock().

2) Add a rcu_read_lock/unlock around the __task_cred() access
   in __sigqueue_alloc()

This needs to be revisited to remove the remaining users of
read_lock(&tasklist_lock) but that's outside the scope of this patch.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
LKML-Reference: <20091210004703.269843657@linutronix.de>

diff --git a/kernel/signal.c b/kernel/signal.c
index 7331656..f67545f 100644 (file)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -218,13 +218,13 @@ __sigqueue_alloc(int sig, struct task_struct *t, gfp_t flags, int override_rlimi
        struct user_struct *user;
 
        /*
-        * We won't get problems with the target's UID changing under us
-        * because changing it requires RCU be used, and if t != current, the
-        * caller must be holding the RCU readlock (by way of a spinlock) and
-        * we use RCU protection here
+        * Protect access to @t credentials. This can go away when all
+        * callers hold rcu read lock.
         */
+       rcu_read_lock();
        user = get_uid(__task_cred(t)->user);
        atomic_inc(&user->sigpending);
+       rcu_read_unlock();
 
        if (override_rlimit ||
            atomic_read(&user->sigpending) <=