Re: [PATCH RFC] hist lookups

Jiri Olsa
2018-11-06T11:54:41Z

On Mon, Nov 05, 2018 at 08:53:42PM -0800, David Miller wrote:
>
> Jiri,
>
> Because you now run queued_events__queue() lockless with that condvar
> trick, it is possible for top->qe.in to be seen as one past the data[]
> array, this is because the rotate_queues() code goes:
>
> if (++top->qe.in > &top->qe.data[1])
> top->qe.in = &top->qe.data[0];
>
> So for a brief moment top->qe.in is out of range and thus
> perf_top__mmap_read_idx() can try to enqueue to top->qe.data[2]
>
> We can just do:
>
> if (top->qe.in == &top->qe.data[1])
> top->qe.in = &top->qe.data[0];
> else
> top->qe.in = &top->qe.data[1];
>
> Or, make top->qe.in an index, and simply go:
>
> top->qe.in ^= 1;
>
> Either way will fix the bug.

ah right.. I had originaly full mutex around that,
then I switched it off in the last patch and did
not realize this implication.. nice ;-)

does it fix the crash you reported earlier?

thanks,
jirka