Re: [PATCH v2] perf,x86: Fix shared register mutual exclusion enforcement

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jun 20, 2013 at 06:42:54PM +0200, Stephane Eranian wrote:
> 
> This patch fixes a problem with the shared registers mutual
> exclusion code and incremental event scheduling by the
> generic perf_event code.
> 
> There was a bug whereby the mutual exclusion on the shared
> registers was not enforced because of incremental scheduling
> abort due to event constraints. As an example on Intel
> Nehalem, consider the following events:
> 
> group1= L1D_CACHE_LD:E_STATE,OFFCORE_RESPONSE_0:PF_RFO,L1D_CACHE_LD:I_STATE
> group2= L1D_CACHE_LD:I_STATE
> 
> The L1D_CACHE_LD event can only be measured by 2 counters. Yet, there
> are 3 instances here. The first group can be scheduled and is committed.
> Then, the generic code tries to schedule group2 and this fails (because
> there is no more counter to support the 3rd instance of L1D_CACHE_LD).
> But in x86_schedule_events() error path, put_event_contraints() is invoked
> on ALL the events and not just the ones that just failed. That causes the
> "lock" on the shared offcore_response MSR to be released. Yet the first group
> is actually scheduled and is exposed to reprogramming of that shared msr by
> the sibling HT thread. In other words, there is no guarantee on what is
> measured.
> 
> This patch fixes the problem by tagging committed events with the
> PERF_X86_EVENT_COMMITTED tag. In the error path of x86_schedule_events(),
> only the events NOT tagged have their constraint released. The tag
> is eventually removed when the event in descheduled.
> 
> Signed-off-by: Stephane Eranian <eranian@google.com>

OK, so I 'accidentally' read the patch again; and noticed something.

In your case above; the get/put constraints are still fully matched.
That is; the first group, which was successful, will not have done a put
yet. So a subsequent get+put should still leave us with a positive 'ref'
count and not undo things.

Only once these events pass through x86_pmu_del() will they get a final
put and the 'ref' count will drop to 0.

Now the problem seems to be the get/put things don't actually count
properly.

However, if we look at __intel_shared_reg_{get,put}_constraints() there
is a refcount in there; namely era->ref; however we don't appear to
clear reg->alloc based on it.

Should we?