6.5-stable review patch. If anyone has any objections, please let me know.
------------------
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
commit 889c58b3155ff4c8e8671c95daef63d6fabbb6b1 upstream.
Audit of the refcounting turned up that perf_pmu_migrate_context()
fails to migrate the ctx refcount.
Fixes: bd2756811766 ("perf: Rewrite core context handling")
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20230612093539.085862001@xxxxxxxxxxxxx
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
include/linux/perf_event.h | 13 ++++++++-----
kernel/events/core.c | 17 +++++++++++++++++
2 files changed, 25 insertions(+), 5 deletions(-)
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -843,11 +843,11 @@ struct perf_event {
};
/*
- * ,-----------------------[1:n]----------------------.
- * V V
- * perf_event_context <-[1:n]-> perf_event_pmu_context <--- perf_event
- * ^ ^ | |
- * `--------[1:n]---------' `-[n:1]-> pmu <-[1:n]-'
+ * ,-----------------------[1:n]------------------------.
+ * V V
+ * perf_event_context <-[1:n]-> perf_event_pmu_context <-[1:n]- perf_event
+ * | |
+ * `--[n:1]-> pmu <-[1:n]--'
*
*
* struct perf_event_pmu_context lifetime is refcount based and RCU freed
@@ -865,6 +865,9 @@ struct perf_event {
* ctx->mutex pinning the configuration. Since we hold a reference on
* group_leader (through the filedesc) it can't go away, therefore it's
* associated pmu_ctx must exist and cannot change due to ctx->mutex.
+ *
+ * perf_event holds a refcount on perf_event_context
+ * perf_event holds a refcount on perf_event_pmu_context
*/
struct perf_event_pmu_context {
struct pmu *pmu;
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4816,6 +4816,11 @@ find_get_pmu_context(struct pmu *pmu, st
void *task_ctx_data = NULL;
if (!ctx->task) {
+ /*
+ * perf_pmu_migrate_context() / __perf_pmu_install_event()
+ * relies on the fact that find_get_pmu_context() cannot fail
+ * for CPU contexts.
+ */
struct perf_cpu_pmu_context *cpc;
cpc = per_cpu_ptr(pmu->cpu_pmu_context, event->cpu);
@@ -12888,6 +12893,9 @@ static void __perf_pmu_install_event(str
int cpu, struct perf_event *event)
{
struct perf_event_pmu_context *epc;
+ struct perf_event_context *old_ctx = event->ctx;
+
+ get_ctx(ctx); /* normally find_get_context() */
event->cpu = cpu;
epc = find_get_pmu_context(pmu, ctx, event);
@@ -12896,6 +12904,11 @@ static void __perf_pmu_install_event(str
if (event->state >= PERF_EVENT_STATE_OFF)
event->state = PERF_EVENT_STATE_INACTIVE;
perf_install_in_context(ctx, event, cpu);
+
+ /*
+ * Now that event->ctx is updated and visible, put the old ctx.
+ */
+ put_ctx(old_ctx);
}
static void __perf_pmu_install(struct perf_event_context *ctx,
@@ -12934,6 +12947,10 @@ void perf_pmu_migrate_context(struct pmu
struct perf_event_context *src_ctx, *dst_ctx;
LIST_HEAD(events);
+ /*
+ * Since per-cpu context is persistent, no need to grab an extra
+ * reference.
+ */
src_ctx = &per_cpu_ptr(&perf_cpu_context, src_cpu)->ctx;
dst_ctx = &per_cpu_ptr(&perf_cpu_context, dst_cpu)->ctx;
