The following commit has been merged into the perf/core branch of tip:
Commit-ID: 82aad7ff7ac25c8cf09d491ae23b9823f1901486
Gitweb: https://git.kernel.org/tip/82aad7ff7ac25c8cf09d491ae23b9823f1901486
Author: Peter Zijlstra <[email protected]>
AuthorDate: Tue, 04 Oct 2022 12:20:39 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Tue, 04 Oct 2022 13:32:09 +02:00
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) <[email protected]>
---
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 7ef0e98..c379770 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);