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

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Date:   Mon, 05 Nov 2018 20:53:42 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     jolsa@...hat.com
Cc:     acme@...nel.org, linux-kernel@...r.kernel.org, namhyung@...nel.org,
        jolsa@...nel.org
Subject: Re: [PATCH RFC] hist lookups


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.