Messages in this thread | | | Date | Thu, 13 Sep 2018 09:07:40 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] perf: Prevent recursion in ring buffer |
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On Wed, Sep 12, 2018 at 09:33:17PM +0200, Jiri Olsa wrote: > Some of the scheduling tracepoints allow the perf_tp_event > code to write to ring buffer under different cpu than the > code is running on.
ARGH.. that is indeed borken.
> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c > index 4a9937076331..0c976ac414c5 100644 > --- a/kernel/events/ring_buffer.c > +++ b/kernel/events/ring_buffer.c > @@ -101,6 +101,7 @@ static void perf_output_put_handle(struct perf_output_handle *handle) > > out: > preempt_enable(); > + atomic_set(&rb->recursion, 0); > } > > static __always_inline bool > @@ -145,6 +146,12 @@ __perf_output_begin(struct perf_output_handle *handle, > goto out; > } > > + if (atomic_cmpxchg(&rb->recursion, 0, 1) != 0) { > + if (rb->nr_pages) > + local_inc(&rb->lost); > + goto out; > + } > + > handle->rb = rb; > handle->event = event; > > @@ -286,6 +293,7 @@ ring_buffer_init(struct ring_buffer *rb, long watermark, int flags) > rb->overwrite = 1; > > atomic_set(&rb->refcount, 1); > + atomic_set(&rb->recursion, 0); > > INIT_LIST_HEAD(&rb->event_list); > spin_lock_init(&rb->event_lock);
That's not a recursion count, that's a test-and-set spinlock, and you got the memory ordering wrong for that.
Also, we tried very hard to avoid atomic ops in the ring-buffer and you just wrecked that. Worse, you wrecked previously working interrupt nesting output.
Let me have a look at this.
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