Messages in this thread | | | From | Alexander Shishkin <> | Subject | Re: [PATCH v1 4/6] perf: Allow using AUX data in perf samples | Date | Mon, 30 Sep 2019 14:50:55 +0300 |
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Peter Zijlstra <peterz@infradead.org> writes:
> On Fri, Aug 09, 2019 at 03:32:39PM +0300, Alexander Shishkin wrote: >> The other problem is sampling SW events, that would require a ctx->lock >> to prevent racing with event_function_call()s from other cpus, resulting >> in somewhat cringy "if (!in_nmi()) raw_spin_lock(...)", but I don't have >> better idea as to how to handle that. > >> +int perf_pmu_aux_sample_output(struct perf_event *event, >> + struct perf_output_handle *handle, >> + unsigned long size) >> +{ >> + unsigned long flags; >> + int ret; >> + >> + /* >> + * NMI vs IRQ >> + * >> + * Normal ->start()/->stop() callbacks run in IRQ mode in scheduler >> + * paths. If we start calling them in NMI context, they may race with >> + * the IRQ ones, that is, for example, re-starting an event that's just >> + * been stopped. >> + */ >> + if (!in_nmi()) >> + raw_spin_lock_irqsave(&event->ctx->lock, flags); >> + >> + ret = event->pmu->snapshot_aux(event, handle, size); >> + >> + if (!in_nmi()) >> + raw_spin_unlock_irqrestore(&event->ctx->lock, flags); >> + >> + return ret; >> +} > > I'm confused... would not something like: > > unsigned long flags; > > local_irq_save(flags); > ret = event->pmu->snapshot_aux(...); > local_irq_restore(flags); > > return ret; > > Be sufficient? By disabling IRQs we already hold off remote > event_function_call()s. > > Or am I misunderstanding the race here?
No, you're right, disabling IRQs covers our bases.
Thanks, -- Alex
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