signal: Document the RCU protection of ->sighand

__cleanup_sighand() frees sighand without RCU grace period. This is
correct but this looks "obviously buggy" and constantly confuses the
readers, add the comments to explain how this works.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Reviewed-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>

kernel/fork.c

@@ -1022,11 +1022,14 @@ void __cleanup_sighand(struct sighand_struct *sighand)
 {
 	if (atomic_dec_and_test(&sighand->count)) {
 		signalfd_cleanup(sighand);
+		/*
+		 * sighand_cachep is SLAB_DESTROY_BY_RCU so we can free it
+		 * without an RCU grace period, see __lock_task_sighand().
+		 */
 		kmem_cache_free(sighand_cachep, sighand);
 	}
 }
 
-
 /*
  * Initialize POSIX timer handling for a thread group.
  */

kernel/signal.c

@@ -1275,7 +1275,17 @@ struct sighand_struct *__lock_task_sighand(struct task_struct *tsk,
 			local_irq_restore(*flags);
 			break;
 		}
-
+		/*
+		 * This sighand can be already freed and even reused, but
+		 * we rely on SLAB_DESTROY_BY_RCU and sighand_ctor() which
+		 * initializes ->siglock: this slab can't go away, it has
+		 * the same object type, ->siglock can't be reinitialized.
+		 *
+		 * We need to ensure that tsk->sighand is still the same
+		 * after we take the lock, we can race with de_thread() or
+		 * __exit_signal(). In the latter case the next iteration
+		 * must see ->sighand == NULL.
+		 */
 		spin_lock(&sighand->siglock);
 		if (likely(sighand == tsk->sighand)) {
 			rcu_read_unlock();