arch/x86/events/rapl.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)
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.
Fixes: 65661f96d3b3 ("perf/x86: Add RAPL hrtimer support")
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
---
Changes in v2:
- Move hrtimer_cancel() to the end of rapl_pmu_event_stop() function.
arch/x86/events/rapl.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
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);
}
--
2.17.1
* Duoming Zhou <duoming@zju.edu.cn> 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.
>
> Fixes: 65661f96d3b3 ("perf/x86: Add RAPL hrtimer support")
> Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
> ---
> Changes in v2:
> - Move hrtimer_cancel() to the end of rapl_pmu_event_stop() function.
>
> arch/x86/events/rapl.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> 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;
> + }
Looks racy now: AFAICS now it's possible for rapl_hrtimer_handle() to
execute at an arbitrary moment after pmu->lock is dropped - which could be
use-after-free after cleanup_rapl_pmus() executes and the PMU is freed,
right?
There's also the quality-of-implementation issue of the hrtimer executing
in a delayed fashion for the *next* event that may have been added, leading
to possibly unexpected results.
Thanks,
Ingo
Hello,
On Wed, 21 Sep 2022 09:53:04 +0200 Ingo Molnar wrote:
> * Duoming Zhou <duoming@zju.edu.cn> 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.
> >
> > Fixes: 65661f96d3b3 ("perf/x86: Add RAPL hrtimer support")
> > Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
> > ---
> > Changes in v2:
> > - Move hrtimer_cancel() to the end of rapl_pmu_event_stop() function.
> >
> > arch/x86/events/rapl.c | 7 +++++--
> > 1 file changed, 5 insertions(+), 2 deletions(-)
> >
> > 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;
> > + }
>
> Looks racy now: AFAICS now it's possible for rapl_hrtimer_handle() to
> execute at an arbitrary moment after pmu->lock is dropped - which could be
> use-after-free after cleanup_rapl_pmus() executes and the PMU is freed,
> right?
>
> There's also the quality-of-implementation issue of the hrtimer executing
> in a delayed fashion for the *next* event that may have been added, leading
> to possibly unexpected results.
Thank your for your suggestions! In order to solve the above problems,
I come up with the following solution.
diff --git a/arch/x86/events/rapl.c b/arch/x86/events/rapl.c
index 77e3a47af5a..a526a08ee6e 100644
--- a/arch/x86/events/rapl.c
+++ b/arch/x86/events/rapl.c
@@ -219,11 +219,13 @@ static enum hrtimer_restart rapl_hrtimer_handle(struct hrtimer *hrtimer)
struct perf_event *event;
unsigned long flags;
- if (!pmu->n_active)
- return HRTIMER_NORESTART;
-
raw_spin_lock_irqsave(&pmu->lock, flags);
+ if (!pmu->n_active) {
+ raw_spin_unlock_irqrestore(&pmu->lock, flags);
+ return HRTIMER_NORESTART;
+ }
+
list_for_each_entry(event, &pmu->active_list, active_entry)
rapl_event_update(event);
@@ -282,7 +284,7 @@ static void rapl_pmu_event_stop(struct perf_event *event, int mode)
WARN_ON_ONCE(pmu->n_active <= 0);
pmu->n_active--;
if (pmu->n_active == 0)
- hrtimer_cancel(&pmu->hrtimer);
+ hrtimer_try_to_cancel(&pmu->hrtimer);
list_del(&event->active_entry);
Firstly, the deadlock could be mitigated. Because if the timer callback function
is running, the hrtimer_try_to_cancel() will directly return.
Secondly, the race could be avoided. Because we use pmu->lock to synchronize and
move the check "if (!pmu->n_active)" into the protection scope of pmu->lock.
If the rapl_pmu_event_stop() has finished, the "pmu->n_active" equals to 0 and
the rapl_hrtimer_handle() will return "HRTIMER_NORESTART".
Thirdly, this solution will not cause quality-of-implementation issue of the hrtimer.
Best regards,
Duoming Zhou
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