[PATCH v6 2/3] bpf: Have __bpf_trace_run() use rcu_read_lock_dont_migrate()

[Steven Rostedt <rostedt@kernel.org>]

From: Steven Rostedt <rostedt@goodmis.org>

In order to switch the protection of tracepoint callbacks from
preempt_disable() to srcu_read_lock_fast() the BPF callback from
tracepoints needs to have migration prevention as the BPF programs expect
to stay on the same CPU as they execute. Put together the RCU protection
with migration prevention and use rcu_read_lock_dont_migrate() in
__bpf_trace_run(). This will allow tracepoints callbacks to be
preemptible.

Link: https://lore.kernel.org/all/CAADnVQKvY026HSFGOsavJppm3-Ajm-VsLzY-OeFUe+BaKMRnDg@mail.gmail.com/

Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 kernel/trace/bpf_trace.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index fe28d86f7c35..abbf0177ad20 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -2062,7 +2062,7 @@ void __bpf_trace_run(struct bpf_raw_tp_link *link, u64 *args)
 	struct bpf_run_ctx *old_run_ctx;
 	struct bpf_trace_run_ctx run_ctx;
 
-	cant_sleep();
+	rcu_read_lock_dont_migrate();
 	if (unlikely(this_cpu_inc_return(*(prog->active)) != 1)) {
 		bpf_prog_inc_misses_counter(prog);
 		goto out;
@@ -2071,13 +2071,12 @@ void __bpf_trace_run(struct bpf_raw_tp_link *link, u64 *args)
 	run_ctx.bpf_cookie = link->cookie;
 	old_run_ctx = bpf_set_run_ctx(&run_ctx.run_ctx);
 
-	rcu_read_lock();
 	(void) bpf_prog_run(prog, args);
-	rcu_read_unlock();
 
 	bpf_reset_run_ctx(old_run_ctx);
 out:
 	this_cpu_dec(*(prog->active));
+	rcu_read_unlock_migrate();
 }
 
 #define UNPACK(...)			__VA_ARGS__
-- 
2.51.0