Messages in this thread Patch in this message | | | Date | Mon, 19 Sep 2022 23:16:27 +0800 (GMT+08:00) | From | duoming@zju ... | Subject | Re: [PATCH] perf/x86/rapl: fix deadlock in rapl_pmu_event_stop |
| |
Hello,
On Mon, 19 Sep 2022 13:45:38 +0200 Peter Zijlstra wrote:
> 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.
Thank you for your time and suggestions! I come up with another solution that will not break the atomicity, the detail is shown below:
diff --git a/arch/x86/events/rapl.c b/arch/x86/events/rapl.c index 77e3a47af5a..7c110092c83 100644 --- a/arch/x86/events/rapl.c +++ b/arch/x86/events/rapl.c @@ -281,8 +281,6 @@ 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) - hrtimer_cancel(&pmu->hrtimer); list_del(&event->active_entry); @@ -300,6 +298,11 @@ static void rapl_pmu_event_stop(struct perf_event *event, int mode) hwc->state |= PERF_HES_UPTODATE; } + if (!pmu->n_active) { + raw_spin_unlock_irqrestore(&pmu->lock, flags); + hrtimer_cancel(&pmu->hrtimer); + return; + } raw_spin_unlock_irqrestore(&pmu->lock, flags); }
I move the hrtimer_cancel() to the end of the rapl_pmu_event_stop() function. As a result, the atomicity will not break and the deadlock bug could be mitigated. > > + } > > > > 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?
I found this bug through a static analysis tool written by myself.
Thanks you!
Best regards, Duoming Zhou
| |