Re: [PATCH] perf/x86/rapl: fix deadlock in rapl_pmu_event_stop

[Peter Zijlstra]

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@xxxxxxxxxx>
> ---
>  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?