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SubjectRe: [PATCH 4/2] perf_counter: Fix swcounter context invariance
On Thu, Aug 13, 2009 at 09:51:55AM +0200, Peter Zijlstra wrote:
> Not really related to this topic, but it needs posting anyway.
>
> ---
>
> Subject: perf_counter: Fix swcounter context invariance
> From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date: Fri Aug 07 13:29:13 CEST 2009
>
> perf_swcounter_is_counting() uses a lock, which means we cannot use
> swcounters from NMI or when holding that particular lock, this is
> unintended.
>
> The below removes the lock, this opens up race window, but not worse
> than the swcounters already experience due to RCU traversal of the
> context in perf_swcounter_ctx_event().
>
> Cc: Paul Mackerras <paulus@samba.org>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>



As a side effect, it's possible this also fixes the hard lockups
while opening a lockdep tracepoint counter.


> ---
> kernel/perf_counter.c | 44 ++++++++++++++++++--------------------------
> 1 file changed, 18 insertions(+), 26 deletions(-)
>
> Index: linux-2.6/kernel/perf_counter.c
> ===================================================================
> --- linux-2.6.orig/kernel/perf_counter.c
> +++ linux-2.6/kernel/perf_counter.c
> @@ -3519,40 +3519,32 @@ static void perf_swcounter_add(struct pe
>
> static int perf_swcounter_is_counting(struct perf_counter *counter)
> {
> - struct perf_counter_context *ctx;
> - unsigned long flags;
> - int count;
> -
> + /*
> + * The counter is active, we're good!
> + */
> if (counter->state == PERF_COUNTER_STATE_ACTIVE)
> return 1;
>
> + /*
> + * The counter is off/error, not counting.
> + */
> if (counter->state != PERF_COUNTER_STATE_INACTIVE)
> return 0;
>
> /*
> - * If the counter is inactive, it could be just because
> - * its task is scheduled out, or because it's in a group
> - * which could not go on the PMU. We want to count in
> - * the first case but not the second. If the context is
> - * currently active then an inactive software counter must
> - * be the second case. If it's not currently active then
> - * we need to know whether the counter was active when the
> - * context was last active, which we can determine by
> - * comparing counter->tstamp_stopped with ctx->time.
> - *
> - * We are within an RCU read-side critical section,
> - * which protects the existence of *ctx.
> + * The counter is inactive, if the context is active
> + * we're part of a group that didn't make it on the 'pmu',
> + * not counting.
> */
> - ctx = counter->ctx;
> - spin_lock_irqsave(&ctx->lock, flags);
> - count = 1;
> - /* Re-check state now we have the lock */
> - if (counter->state < PERF_COUNTER_STATE_INACTIVE ||
> - counter->ctx->is_active ||
> - counter->tstamp_stopped < ctx->time)
> - count = 0;
> - spin_unlock_irqrestore(&ctx->lock, flags);
> - return count;
> + if (counter->ctx->is_active)
> + return 0;
> +
> + /*
> + * We're inactive and the context is too, this means the
> + * task is scheduled out, we're counting events that happen
> + * to us, like migration events.
> + */
> + return 1;
> }
>
> static int perf_swcounter_match(struct perf_counter *counter,
>
> --
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