linux-kernel - perf_events: question about __perf_event

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Message-ID: <AANLkTikX=tQwYKj9VZhCNudErf4TP-74+Jv3WPvSdUMn@mail.gmail.com>
Date:	Fri, 21 Jan 2011 19:06:18 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>, mingo@...e.hu,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: perf_events: question about __perf_event_read()

Hi,

I think the code below still has a problem in case of a per-cpu event.

If you issue a read() on a different CPU, then you IPI to the event's cpu.
By the time you get there, the event may be de-scheduled in which
case you don't want to issue event->pmu_read() nor update context
timings. The function has a test but it seems to be checking the per-cpu
case only.

I have seen panics on P4 with this code because it goes all the way
down to rdmsrl() with a bogus counter index (like -1).

Am I missing something here?

static void __perf_event_read(void *info)
{
        struct perf_event *event = info;
        struct perf_event_context *ctx = event->ctx;
        struct perf_cpu_context *cpuctx = __get_cpu_context(ctx);

        /*
         * If this is a task context, we need to check whether it is
         * the current task context of this cpu.  If not it has been
         * scheduled out before the smp call arrived.  In that case
         * event->count would have been updated to a recent sample
         * when the event was scheduled out.
         */
        if (ctx->task && cpuctx->task_ctx != ctx)
                return;

        raw_spin_lock(&ctx->lock);
        update_context_time(ctx);
        update_event_times(event);
        raw_spin_unlock(&ctx->lock);

        event->pmu->read(event);
}
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