Re: [PATCH 2/2] signal: document the RCU protection of ->sighand

[Steven Rostedt]

On Mon, 22 Sep 2014 18:44:40 +0200
Oleg Nesterov <oleg@xxxxxxxxxx> 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@xxxxxxxxxxx>

-- Steve

> 
> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> ---
>  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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