[PATCH bpf-next v2] bpf: Remove migrate_disable in kprobe_multi_link_prog_run

[Tao Chen <chen.dylane@linux.dev>]

bpf program should run under migration disabled, kprobe_multi_link_prog_run
called all the way from graph tracer, which disables preemption in
function_graph_enter_regs, as Jiri and Yonghong suggested, there is no
need to use migrate_disable. As a result, some overhead maybe will be
reduced.

Fixes: 0dcac2725406 ("bpf: Add multi kprobe link")
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Tao Chen <chen.dylane@linux.dev>
---
 kernel/trace/bpf_trace.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

Change list:
 v1 -> v2:
  - s/called the way/called all the way/.(Jiri)
 v1: https://lore.kernel.org/bpf/f7acfd22-bcf3-4dff-9a87-7c1e6f84ce9c@linux.dev

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 3ae52978cae..5701791e3cb 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -2734,14 +2734,19 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
 		goto out;
 	}
 
-	migrate_disable();
+	/*
+	 * bpf program should run under migration disabled, kprobe_multi_link_prog_run
+	 * called all the way from graph tracer, which disables preemption in
+	 * function_graph_enter_regs, so there is no need to use migrate_disable.
+	 * Accessing the above percpu data bpf_prog_active is also safe for the same
+	 * reason.
+	 */
 	rcu_read_lock();
 	regs = ftrace_partial_regs(fregs, bpf_kprobe_multi_pt_regs_ptr());
 	old_run_ctx = bpf_set_run_ctx(&run_ctx.session_ctx.run_ctx);
 	err = bpf_prog_run(link->link.prog, regs);
 	bpf_reset_run_ctx(old_run_ctx);
 	rcu_read_unlock();
-	migrate_enable();
 
  out:
 	__this_cpu_dec(bpf_prog_active);
-- 
2.48.1