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SubjectRe: [PATCH] perf/x86/rapl: fix deadlock in rapl_pmu_event_stop
On Sat, Sep 17, 2022 at 10:47:29PM +0800, Duoming Zhou wrote:
> There is a deadlock in rapl_pmu_event_stop(), the process is
> shown below:
>
> (thread 1) | (thread 2)
> rapl_pmu_event_stop() | rapl_hrtimer_handle()
> ... | if (!pmu->n_active)
> raw_spin_lock_irqsave() //(1) | ...
> ... |
> hrtimer_cancel() | raw_spin_lock_irqsave() //(2)
> (block forever)
>
> We hold pmu->lock in position (1) and use hrtimer_cancel() to wait
> rapl_hrtimer_handle() to stop, but rapl_hrtimer_handle() also need
> pmu->lock in position (2). As a result, the rapl_pmu_event_stop()
> will be blocked forever.
>
> This patch extracts hrtimer_cancel() from the protection of
> raw_spin_lock_irqsave(). As a result, the rapl_hrtimer_handle() could
> obtain the pmu->lock. In order to prevent race conditions, we put
> "if (!pmu->n_active)" in rapl_hrtimer_handle() under the protection
> of raw_spin_lock_irqsave().
>
> Fixes: 65661f96d3b3 ("perf/x86: Add RAPL hrtimer support")
> Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
> ---
> arch/x86/events/rapl.c | 9 ++++++---
> 1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/events/rapl.c b/arch/x86/events/rapl.c
> index 77e3a47af5a..97c71538d01 100644
> --- a/arch/x86/events/rapl.c
> +++ b/arch/x86/events/rapl.c
> @@ -219,11 +219,11 @@ static enum hrtimer_restart rapl_hrtimer_handle(struct hrtimer *hrtimer)
> struct perf_event *event;
> unsigned long flags;
>
> + raw_spin_lock_irqsave(&pmu->lock, flags);
> +
> if (!pmu->n_active)
> return HRTIMER_NORESTART;

Except now you return with the lock held...

>
> - raw_spin_lock_irqsave(&pmu->lock, flags);
> -
> list_for_each_entry(event, &pmu->active_list, active_entry)
> rapl_event_update(event);
>
> @@ -281,8 +281,11 @@ static void rapl_pmu_event_stop(struct perf_event *event, int mode)
> if (!(hwc->state & PERF_HES_STOPPED)) {
> WARN_ON_ONCE(pmu->n_active <= 0);
> pmu->n_active--;
> - if (pmu->n_active == 0)
> + if (!pmu->n_active) {
> + raw_spin_unlock_irqrestore(&pmu->lock, flags);
> hrtimer_cancel(&pmu->hrtimer);
> + raw_spin_lock_irqsave(&pmu->lock, flags);

Doing a lock-break makes the nr_active and list_del thing non-atomic,
breaking the whole purpose of the lock.

> + }
>
> list_del(&event->active_entry);


Now; did you actually observe this deadlock or is this a code-reading
exercise? If you saw an actual deadlock, was cpu-hotplug involved?


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