Commit-ID: 392809b25833548ccfc55e61b76c8451a5073216 Gitweb: http://git.kernel.org/tip/392809b25833548ccfc55e61b76c8451a5073216 Author: Oleg Nesterov <oleg@xxxxxxxxxx> AuthorDate: Sun, 28 Sep 2014 23:44:18 +0200 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> CommitDate: Wed, 29 Oct 2014 10:07:18 -0700 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@xxxxxxxxxx> Reviewed-by: Steven Rostedt <rostedt@xxxxxxxxxxx> Reviewed-by: Rik van Riel <riel@xxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Reviewed-by: Pranith Kumar <bobby.prani@xxxxxxxxx> --- kernel/fork.c | 5 ++++- kernel/signal.c | 12 +++++++++++- 2 files changed, 15 insertions(+), 2 deletions(-) diff --git a/kernel/fork.c b/kernel/fork.c index 9b7d746..9ca8418 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 5482098..19e3513 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(); -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html