6.6-stable review patch. If anyone has any objections, please let me know.
------------------
From: Luo Gengkun luogengkun@huaweicloud.com
[ Upstream commit 2016066c66192a99d9e0ebf433789c490a6785a2 ]
Syskaller triggers a warning due to prev_epc->pmu != next_epc->pmu in perf_event_swap_task_ctx_data(). vmcore shows that two lists have the same perf_event_pmu_context, but not in the same order.
The problem is that the order of pmu_ctx_list for the parent is impacted by the time when an event/PMU is added. While the order for a child is impacted by the event order in the pinned_groups and flexible_groups. So the order of pmu_ctx_list in the parent and child may be different.
To fix this problem, insert the perf_event_pmu_context to its proper place after iteration of the pmu_ctx_list.
The follow testcase can trigger above warning:
# perf record -e cycles --call-graph lbr -- taskset -c 3 ./a.out & # perf stat -e cpu-clock,cs -p xxx // xxx is the pid of a.out
test.c
void main() { int count = 0; pid_t pid;
printf("%d running\n", getpid()); sleep(30); printf("running\n");
pid = fork(); if (pid == -1) { printf("fork error\n"); return; } if (pid == 0) { while (1) { count++; } } else { while (1) { count++; } } }
The testcase first opens an LBR event, so it will allocate task_ctx_data, and then open tracepoint and software events, so the parent context will have 3 different perf_event_pmu_contexts. On inheritance, child ctx will insert the perf_event_pmu_context in another order and the warning will trigger.
[ mingo: Tidied up the changelog. ]
Fixes: bd2756811766 ("perf: Rewrite core context handling") Signed-off-by: Luo Gengkun luogengkun@huaweicloud.com Signed-off-by: Ingo Molnar mingo@kernel.org Reviewed-by: Kan Liang kan.liang@linux.intel.com Link: https://lore.kernel.org/r/20250122073356.1824736-1-luogengkun@huaweicloud.co... Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/events/core.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/kernel/events/core.c b/kernel/events/core.c index 5d6458ea675e9..18f0a3aa6d7c6 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -4842,7 +4842,7 @@ static struct perf_event_pmu_context * find_get_pmu_context(struct pmu *pmu, struct perf_event_context *ctx, struct perf_event *event) { - struct perf_event_pmu_context *new = NULL, *epc; + struct perf_event_pmu_context *new = NULL, *pos = NULL, *epc; void *task_ctx_data = NULL;
if (!ctx->task) { @@ -4899,12 +4899,19 @@ find_get_pmu_context(struct pmu *pmu, struct perf_event_context *ctx, atomic_inc(&epc->refcount); goto found_epc; } + /* Make sure the pmu_ctx_list is sorted by PMU type: */ + if (!pos && epc->pmu->type > pmu->type) + pos = epc; }
epc = new; new = NULL;
- list_add(&epc->pmu_ctx_entry, &ctx->pmu_ctx_list); + if (!pos) + list_add_tail(&epc->pmu_ctx_entry, &ctx->pmu_ctx_list); + else + list_add(&epc->pmu_ctx_entry, pos->pmu_ctx_entry.prev); + epc->ctx = ctx;
found_epc: