From 82aad7ff7ac25c8cf09d491ae23b9823f1901486 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Tue, 4 Oct 2022 12:20:39 +0200 Subject: [PATCH] perf/hw_breakpoint: Annotate tsk->perf_event_mutex vs ctx->mutex Perf fuzzer gifted a lockdep splat: perf_event_init_context() mutex_lock(parent_ctx->mutex); (B) inherit_task_group() inherit_group() inherit_event() perf_event_alloc() perf_try_init_event() := hw_breakpoint_event_init() register_perf_hw_breakpoint() mutex_lock(child->perf_event_mutex); (A) Which is against the normal (documented) order. Now, this is a false positive in that child is not published yet, but also inherited events never end up on ->perf_event_list. Annotate this one away. Fixes: 0912037fec11 ("perf/hw_breakpoint: Reduce contention with large number of tasks") Signed-off-by: Peter Zijlstra (Intel) --- kernel/events/hw_breakpoint.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c index 7ef0e98d31e2..c3797701339c 100644 --- a/kernel/events/hw_breakpoint.c +++ b/kernel/events/hw_breakpoint.c @@ -117,7 +117,17 @@ static struct mutex *bp_constraints_lock(struct perf_event *bp) struct mutex *tsk_mtx = get_task_bps_mutex(bp); if (tsk_mtx) { - mutex_lock(tsk_mtx); + /* + * Fully analogous to the perf_try_init_event() nesting + * argument in the comment near perf_event_ctx_lock_nested(); + * this child->perf_event_mutex cannot ever deadlock against + * the parent->perf_event_mutex usage from + * perf_event_task_{en,dis}able(). + * + * Specifically, inherited events will never occur on + * ->perf_event_list. + */ + mutex_lock_nested(tsk_mtx, SINGLE_DEPTH_NESTING); percpu_down_read(&bp_cpuinfo_sem); } else { percpu_down_write(&bp_cpuinfo_sem);