Messages in this thread Patch in this message | | | Date | Mon, 22 Sep 2014 18:44:40 +0200 | From | Oleg Nesterov <> | Subject | [PATCH 2/2] signal: document the RCU protection of ->sighand |
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__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> --- 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; -- 1.5.5.1
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