Re: [PATCH 4/7] perf: Free aux pages in unmap path

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Dec 04, 2015 at 06:02:06PM +0100, Peter Zijlstra wrote:
> The only solution I've come up with is:
> 
> 	struct rb_aux *aux = rb->aux;
> 
> 	if (aux && vma->vm_pgoff == aux->pgoff) {
> 		ctx = perf_event_ctx_lock(event);

Can't do this at all, see the comment in put_event(). perf_read_group()
accesses user memory (and hence causes faults, which in turn take
mmap_sem) while holding ctx::mutex.

So neither this, not what you proposed can work.

Will need moar thinking.

> 		if (!atomic_dec_and_mutex_lock(&aux->mmap_count, &event->mmap_mutex) {
> 			/* we now hold both ctx::mutex and event::mmap_mutex */
> 			rb->aux = NULL;
> 			ring_buffer_put(rb); /* aux had a reference */
> 			_perf_event_stop(event);
> 			ring_buffer_put_aux(aux); /* should be last */
> 			mutex_unlock(&event->mmap_mutex);
> 		}
> 		mutex_unlock(&ctx->mutex);
> 	}
> 
>