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Subject[PATCH] perf/core: Avoid freeing static PMU contexts when PMU is unregistered
Since commit 1fd7e4169954 ("perf/core: Remove perf_cpu_context::unique_pmu"),
when a PMU is unregistered then its associated ->pmu_cpu_context is
unconditionally freed. Whilst this is fine for dynamically allocated
context types (i.e. those registered using perf_invalid_context), this
causes a problem for sharing of static contexts such as
perf_{sw,hw}_context, which are used by multiple built-in PMUs and
effectively have a global lifetime.

Whilst testing the ARM SPE driver, which must use perf_sw_context to
support per-task AUX tracing, unregistering the driver as a result of a
module unload resulted in:

Unable to handle kernel NULL pointer dereference at virtual address 00000038
Internal error: Oops: 96000004 [#1] PREEMPT SMP
Modules linked in: [last unloaded: arm_spe_pmu]
PC is at ctx_resched+0x38/0xe8
LR is at perf_event_exec+0x20c/0x278

since the software context has been freed and the ctx.pmu->pmu_disable_count
field has been set to NULL.

This patch fixes the problem by avoiding the freeing of static PMU contexts
altogether. Whilst the sharing of dynamic contexts is questionable, this
actually requires the caller to share their context pointer explicitly
and so the burden is on them to manage the object lifetime.

Cc: Peter Zijlstra <>
Fixes: 1fd7e4169954 ("perf/core: Remove perf_cpu_context::unique_pmu")
Reported-by: Kim Phillips <>
Signed-off-by: Will Deacon <>
kernel/events/core.c | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 6bc21e202ae4..243bfc68d0fe 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8955,6 +8955,14 @@ static struct perf_cpu_context __percpu *find_pmu_context(int ctxn)

static void free_pmu_context(struct pmu *pmu)
+ /*
+ * Static contexts such as perf_sw_context have a global lifetime
+ * and may be shared between different PMUs. Avoid freeing them
+ * when a single PMU is going away.
+ */
+ if (pmu->task_ctx_nr > perf_invalid_context)
+ return;
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