Hi,
I'm currently integrating user-space performance counters from
Perf into LTTng-UST, and I'm noticing something odd regarding
the home-made sequence lock found at:
kernel/events/core.c: perf_event_update_userpage()
++userpg->lock;
barrier();
[...]
barrier();
++userpg->lock;
This goes in pair with something like this at user-level:
do {
seq = pc->lock;
barrier();
idx = pc->index;
count = pc->offset;
if (idx)
count += rdpmc(idx - 1);
barrier();
} while (pc->lock != seq);
As we see, only compiler barrier() are protecting all this.
First question, is it possible that the update be performed
by a thread running on a different CPU than the thread reading
the info in user-space ?
I would be tempted to use a volatile semantic on all reads of the
lock field (ACCESS_ONCE()). Secondly, read sequence locks usually use a
smp_rmb() at the end of the seqcount_begin(), and at the beginning
of the seqcount_retry(). Moreover, this is usually matched
by smp_wmb() in write_seqcount begin/end().
Am I missing something special about this lock that makes these
barriers unnecessary ?
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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