Messages in this thread | | | Date | Mon, 22 Sep 2014 15:00:58 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH 2/2] signal: document the RCU protection of ->sighand |
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On Mon, 22 Sep 2014 18:44:40 +0200 Oleg Nesterov <oleg@redhat.com> wrote:
> __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.
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
-- Steve
> > Signed-off-by: Oleg Nesterov <oleg@redhat.com> > --- > 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 1380d8a..2dd9f1d 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -1017,11 +1017,14 @@ 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 61a1f55..9562a4f 100644 > --- a/kernel/signal.c > +++ b/kernel/signal.c > @@ -1272,7 +1272,17 @@ struct sighand_struct *__lock_task_sighand(struct task_struct *tsk, > sighand = rcu_dereference(tsk->sighand); > if (unlikely(sighand == NULL)) > 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_irqsave(&sighand->siglock, *flags); > if (likely(sighand == tsk->sighand)) > break;
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