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);
> }
>
>
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