6.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Puranjay Mohan commit 3d8d74100954a3b17e5c5e37adfe14e16b1db103 upstream. perf_pmu_sched_task() returns early when cpuctx->task_ctx is set and leaves the work to perf_ctx_sched_task_cb(), which only walks ctx->pmu_ctx_list. A PMU whose events are all CPU-wide is not on that list, so nothing calls its sched_task(). With perf record -b -e cycles -a -- ls armv8pmu_sched_task() is skipped on every switch to a task that has a perf context but no event on that PMU, and BRBE records leak across the task boundary. intel_pmu_lbr_add() calls perf_sched_cb_inc() unconditionally too, so LBR records leak the same way on x86. Drop the early return and skip only the CPCs that perf_ctx_sched_task_cb() handles. That one needs a gate of its own to make the split exact: it tests cpc->sched_cb_usage, which perf_sched_cb_inc() sets per CPU for every branch stack user, so a task with an event for that PMU pinned to another CPU would be handled twice. On x86 the second __intel_pmu_lbr_restore() finds lbr_stack_state == LBR_NONE and calls intel_pmu_lbr_reset(), throwing away the callstack the first one restored. cpc->task_epc is set only while a task context is scheduled in, and there is one epc per PMU on ctx->pmu_ctx_list, so the two gates are inverses. For the CPCs perf_pmu_sched_task() picks up, the callback now runs outside the perf_ctx_disable() and perf_ctx_enable() pair in perf_event_context_sched_in(). __perf_pmu_sched_task() disables the PMU around the call itself. Fixes: bd2756811766 ("perf: Rewrite core context handling") Signed-off-by: Puranjay Mohan Signed-off-by: Peter Zijlstra (Intel) Tested-by: Yifan Wu Link: https://patch.msgid.link/20260810133540.1947118-3-puranjay@kernel.org Cc: stable@vger.kernel.org Signed-off-by: Greg Kroah-Hartman --- kernel/events/core.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -3655,6 +3655,9 @@ static void perf_ctx_sched_task_cb(struc list_for_each_entry(pmu_ctx, &ctx->pmu_ctx_list, pmu_ctx_entry) { cpc = this_cpc(pmu_ctx->pmu); + if (cpc->task_epc != pmu_ctx) + continue; + if (cpc->sched_cb_usage && pmu_ctx->pmu->sched_task) pmu_ctx->pmu->sched_task(pmu_ctx, task, sched_in); } @@ -3815,15 +3818,17 @@ static void perf_pmu_sched_task(struct t struct task_struct *next, bool sched_in) { - struct perf_cpu_context *cpuctx = this_cpu_ptr(&perf_cpu_context); struct perf_cpu_pmu_context *cpc, *cpc2; - /* cpuctx->task_ctx will be handled in perf_event_context_sched_in/out */ - if (prev == next || cpuctx->task_ctx) + if (prev == next) return; - list_for_each_entry_safe(cpc, cpc2, this_cpu_ptr(&sched_cb_list), sched_cb_entry) + list_for_each_entry_safe(cpc, cpc2, this_cpu_ptr(&sched_cb_list), sched_cb_entry) { + if (cpc->task_epc) + continue; + __perf_pmu_sched_task(cpc, sched_in ? next : prev, sched_in); + } } static void perf_event_switch(struct task_struct *task,