Re: [PATCHv2 3/4] arm-cci: Add routines to enable/disable all counters

From: Suzuki K. Poulose
Date: Thu Nov 05 2015 - 12:52:46 EST

On 05/11/15 17:27, Mark Rutland wrote:
Can we not get rid of the mask entirely? The combination of used_mask
and each event's hwc->state tells us which counters are actually in use.

The problem is that neither hwc->state nor the cci_pmu->hw_events->events is
protected by pmu_lock, while enable/disable counter is. So we cannot really
rely on ((struct perf_event *)(cci_pmu->hw_events->events[counter]))->hw->state.

They must be protected somehow, or we'd have races against cross-calls
and/or the interrupt handler.

Are we protected due to being cpu-affine with interrupts disabled when
modifying these, is there some other mechanism that protects us, or do
we have additional problems here?

Each perf_event is allocated a counter id atomically using the bit mask. So,
once the id is allocated nobody messes with that id from the PMU side. And,
the hw->state may have its own protection within the generic perf layer(which
I haven't checked).


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