From: James Clark james.clark@arm.com
[ Upstream commit 961c39121759ad09a89598ec4ccdd34ae0468a19 ]
When using per-process mode and event inheritance is set to true, forked processes will create a new perf events via inherit_event() -> perf_event_alloc(). But these events will not have ring buffers assigned to them. Any call to wakeup will be dropped if it's called on an event with no ring buffer assigned because that's the object that holds the wakeup list.
If the child event is disabled due to a call to perf_aux_output_begin() or perf_aux_output_end(), the wakeup is dropped leaving userspace hanging forever on the poll.
Normally the event is explicitly re-enabled by userspace after it wakes up to read the aux data, but in this case it does not get woken up so the event remains disabled.
This can be reproduced when using Arm SPE and 'stress' which forks once before running the workload. By looking at the list of aux buffers read, it's apparent that they stop after the fork:
perf record -e arm_spe// -vvv -- stress -c 1
With this patch applied they continue to be printed. This behaviour doesn't happen when using systemwide or per-cpu mode.
Reported-by: Ruben Ayrapetyan Ruben.Ayrapetyan@arm.com Signed-off-by: James Clark james.clark@arm.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Link: https://lkml.kernel.org/r/20211206113840.130802-2-james.clark@arm.com Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/events/core.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-)
diff --git a/kernel/events/core.c b/kernel/events/core.c index 63f0414666438..0b0655f4d0126 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -5934,6 +5934,8 @@ static void ring_buffer_attach(struct perf_event *event, struct perf_buffer *old_rb = NULL; unsigned long flags;
+ WARN_ON_ONCE(event->parent); + if (event->rb) { /* * Should be impossible, we set this when removing @@ -5991,6 +5993,9 @@ static void ring_buffer_wakeup(struct perf_event *event) { struct perf_buffer *rb;
+ if (event->parent) + event = event->parent; + rcu_read_lock(); rb = rcu_dereference(event->rb); if (rb) { @@ -6004,6 +6009,9 @@ struct perf_buffer *ring_buffer_get(struct perf_event *event) { struct perf_buffer *rb;
+ if (event->parent) + event = event->parent; + rcu_read_lock(); rb = rcu_dereference(event->rb); if (rb) { @@ -6703,7 +6711,7 @@ static unsigned long perf_prepare_sample_aux(struct perf_event *event, if (WARN_ON_ONCE(READ_ONCE(sampler->oncpu) != smp_processor_id())) goto out;
- rb = ring_buffer_get(sampler->parent ? sampler->parent : sampler); + rb = ring_buffer_get(sampler); if (!rb) goto out;
@@ -6769,7 +6777,7 @@ static void perf_aux_sample_output(struct perf_event *event, if (WARN_ON_ONCE(!sampler || !data->aux_size)) return;
- rb = ring_buffer_get(sampler->parent ? sampler->parent : sampler); + rb = ring_buffer_get(sampler); if (!rb) return;