[RFC PATCH] trace: prevent preemption in perf_ftrace_function_call()

[王贇]

With CONFIG_DEBUG_PREEMPT we observed reports like:

  BUG: using smp_processor_id() in preemptible
  caller is perf_ftrace_function_call+0x6f/0x2e0
  CPU: 1 PID: 680 Comm: a.out Not tainted
  Call Trace:
   <TASK>
   dump_stack_lvl+0x8d/0xcf
   check_preemption_disabled+0x104/0x110
   ? optimize_nops.isra.7+0x230/0x230
   ? text_poke_bp_batch+0x9f/0x310
   perf_ftrace_function_call+0x6f/0x2e0
   ...
   __text_poke+0x5/0x620
   text_poke_bp_batch+0x9f/0x310

This tell us the CPU could be changed after task is preempted, and
the checking on CPU before preemption is helpless.

This patch just turn off preemption in perf_ftrace_function_call()
to prevent CPU changing.

Signed-off-by: Michael Wang <yun.wang@xxxxxxxxxxxxxxxxx>
---
 kernel/trace/trace_event_perf.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
index 6aed10e..5486b18 100644
--- a/kernel/trace/trace_event_perf.c
+++ b/kernel/trace/trace_event_perf.c
@@ -438,15 +438,17 @@ void perf_trace_buf_update(void *record, u16 type)
 	int rctx;
 	int bit;

+	preempt_disable_notrace();
+
 	if (!rcu_is_watching())
-		return;
+		goto out;

 	if ((unsigned long)ops->private != smp_processor_id())
-		return;
+		goto out;

 	bit = ftrace_test_recursion_trylock(ip, parent_ip);
 	if (bit < 0)
-		return;
+		goto out;

 	event = container_of(ops, struct perf_event, ftrace_ops);

@@ -468,16 +470,18 @@ void perf_trace_buf_update(void *record, u16 type)

 	entry = perf_trace_buf_alloc(ENTRY_SIZE, NULL, &rctx);
 	if (!entry)
-		goto out;
+		goto out_unlock;

 	entry->ip = ip;
 	entry->parent_ip = parent_ip;
 	perf_trace_buf_submit(entry, ENTRY_SIZE, rctx, TRACE_FN,
 			      1, &regs, &head, NULL);

-out:
+out_unlock:
 	ftrace_test_recursion_unlock(bit);
 #undef ENTRY_SIZE
+out:
+	preempt_enable_notrace();
 }

 static int perf_ftrace_function_register(struct perf_event *event)
-- 
1.8.3.1