linux-kernel - Re: [PATCH RFC] hist lookups

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Date:   Tue, 6 Nov 2018 12:54:36 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     David Miller <davem@...emloft.net>
Cc:     acme@...nel.org, linux-kernel@...r.kernel.org, namhyung@...nel.org,
        jolsa@...nel.org
Subject: Re: [PATCH RFC] hist lookups

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