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

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Date:   Mon, 19 Nov 2018 10:12:57 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     David Miller <davem@...emloft.net>, acme@...nel.org,
        linux-kernel@...r.kernel.org, jolsa@...nel.org, kernel-team@....com
Subject: Re: [PATCH RFC] hist lookups

On Mon, Nov 19, 2018 at 02:26:03PM +0900, Namhyung Kim wrote:
> Hi Jirka
> 
> Sorry for late!
> 
> On Tue, Nov 06, 2018 at 12:54:36PM +0100, Jiri Olsa wrote:
> > 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 ;-)
> 
> I like the rotate_queues() using cond-variable.  Have you tried to use
> the same for hists->lock in hists__get_rotate_entries_in() too?
> 
> Eventually it'd be nice to avoid locks when a single thread processes
> all the events.

yep, I thought we could use it there as well, but it could
be more tricky because we use hists->lock for that, which
is used on other places as well.. will check

thanks,
jirka