bpf: Move out synchronize_rcu_tasks_trace from mutex CS

Commit ef1b808e3b7c ("bpf: Fix UAF via mismatching bpf_prog/attachment
RCU flavors") resolved a possible UAF issue in uprobes that attach
non-sleepable bpf prog by explicitly waiting for a tasks-trace-RCU grace
period. But, in the current implementation, synchronize_rcu_tasks_trace
is included within the mutex critical section, which increases the
length of the critical section and may affect performance. So let's move
out synchronize_rcu_tasks_trace from mutex CS.

Signed-off-by: Pu Lehui <[e-mail]>
Reviewed-by: Jiri Olsa <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

kernel/trace/bpf_trace.c

···
 {
 	struct bpf_prog_array *old_array;
 	struct bpf_prog_array *new_array;
+	struct bpf_prog *prog = NULL;
 	int ret;
 
 	mutex_lock(&bpf_event_mutex);
···
 	}
 
 put:
-	/*
-	 * It could be that the bpf_prog is not sleepable (and will be freed
-	 * via normal RCU), but is called from a point that supports sleepable
-	 * programs and uses tasks-trace-RCU.
-	 */
-	synchronize_rcu_tasks_trace();
-
-	bpf_prog_put(event->prog);
+	prog = event->prog;
 	event->prog = NULL;
 
 unlock:
 	mutex_unlock(&bpf_event_mutex);
+
+	if (prog) {
+		/*
+		 * It could be that the bpf_prog is not sleepable (and will be freed
+		 * via normal RCU), but is called from a point that supports sleepable
+		 * programs and uses tasks-trace-RCU.
+		 */
+		synchronize_rcu_tasks_trace();
+
+		bpf_prog_put(prog);
+	}
 }
 
 int perf_event_query_prog_array(struct perf_event *event, void __user *info)