Re: ring_buffer_attach && cond_synchronize_rcu (Was: percpu-rwsem: Optimize readers and reduce global impact)

[Oleg Nesterov]

On 06/16, Peter Zijlstra wrote:
>
>
> I made that the below.
>
> Are you OK with the Changelog edits and the added SoB ?

Of course,

Thanks Peter!

Oleg.

> ---
> Subject: perf: Fix ring_buffer_attach() RCU sync, again.
> From: Oleg Nesterov <oleg@xxxxxxxxxx>
> Date: Sat, 30 May 2015 22:04:25 +0200
>
> While looking for other users of get_state/cond_sync. I Found
> ring_buffer_attach() and it looks obviously buggy?
>
> Don't we need to ensure that we have "synchronize" _between_
> list_del() and list_add() ?
>
> IOW. Suppose that ring_buffer_attach() preempts right_after
> get_state_synchronize_rcu() and gp completes before spin_lock().
>
> In this case cond_synchronize_rcu() does nothing and we reuse
> ->rb_entry without waiting for gp in between?
>
> It also moves the ->rcu_pending check under "if (rb)", to make it
> more readable imo.
>
> Cc: tj@xxxxxxxxxx
> Cc: mingo@xxxxxxxxxx
> Cc: der.herr@xxxxxxx
> Cc: dave@xxxxxxxxxxxx
> Cc: torvalds@xxxxxxxxxxxxxxxxxxxx
> Cc: josh@xxxxxxxxxxxxxxxx
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> Fixes: b69cf53640da ("perf: Fix a race between ring_buffer_detach() and ring_buffer_attach()")
> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Link: http://lkml.kernel.org/r/20150530200425.GA15748@xxxxxxxxxx
> ---
>  kernel/events/core.c |   14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
>
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4307,20 +4307,20 @@ static void ring_buffer_attach(struct pe
>  		WARN_ON_ONCE(event->rcu_pending);
>
>  		old_rb = event->rb;
> -		event->rcu_batches = get_state_synchronize_rcu();
> -		event->rcu_pending = 1;
> -
>  		spin_lock_irqsave(&old_rb->event_lock, flags);
>  		list_del_rcu(&event->rb_entry);
>  		spin_unlock_irqrestore(&old_rb->event_lock, flags);
> -	}
>
> -	if (event->rcu_pending && rb) {
> -		cond_synchronize_rcu(event->rcu_batches);
> -		event->rcu_pending = 0;
> +		event->rcu_batches = get_state_synchronize_rcu();
> +		event->rcu_pending = 1;
>  	}
>
>  	if (rb) {
> +		if (event->rcu_pending) {
> +			cond_synchronize_rcu(event->rcu_batches);
> +			event->rcu_pending = 0;
> +		}
> +
>  		spin_lock_irqsave(&rb->event_lock, flags);
>  		list_add_rcu(&event->rb_entry, &rb->event_list);
>  		spin_unlock_irqrestore(&rb->event_lock, flags);

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