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

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Message-ID: <20181119052603.GA20153@sejong>
Date:   Mon, 19 Nov 2018 14:26:03 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Jiri Olsa <jolsa@...hat.com>
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

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.

Thanks,
Namhyung