Re: [PATCH] perf/x86: Prevent NULL event deref in handle_pmi_common()

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Dec 12, 2025 at 04:49:43PM +0800, evan.li@linux.alibaba.com wrote:
> From: Evan Li <evan.li@linux.alibaba.com>
> 
> handle_pmi_common() may observe an active bit set in cpuc->active_mask
> while the corresponding cpuc->events[] entry has already been cleared,
> which leads to a NULL pointer dereference.
> 
> This can happen when interrupt throttling stops all events in a group
> while PEBS processing is still in progress. perf_event_overflow() can
> trigger perf_event_throttle_group(), which stops the group and clears
> the cpuc->events[] entry, but the active bit may still be set when
> handle_pmi_common() iterates over the events.
> 
> The following change:
> 
> 7e772a93 ("perf/x86: Fix NULL event access and potential PEBS record loss")
> 
> moved cpuc->events[] clearing from x86_pmu_stop() to x86_pmu_del() and
> relied on cpuc->active_mask/pebs_enabled checks. However,
> handle_pmi_common() can still encounter a NULL cpuc->events[] entry
> despite the active bit being set.

How? What is doing del() concurrently with the pmi?