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>

2 files changed, 15 insertions, 2 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 9b7d746d6d62..9ca84189cfc2 100644
--- a/kernel/fork.c
+++ b/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.
  */
diff --git a/kernel/signal.c b/kernel/signal.c
index 54820984a872..19e35135fc60 100644
--- a/kernel/signal.c
+++ b/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();