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>

1 files changed, 4 insertions, 1 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.
  */