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Commit 392809b2 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Paul E. McKenney
Browse files

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: default avatarOleg Nesterov <oleg@redhat.com>
Reviewed-by: default avatarSteven Rostedt <rostedt@goodmis.org>
Reviewed-by: default avatarRik van Riel <riel@redhat.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: default avatarPranith Kumar <bobby.prani@gmail.com>
parent eca1a089

kernel/fork.c

+4 −1
Original line number Diff line number Diff line
@@ -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

+11 −1
Original line number Diff line number Diff line
@@ -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();