[PATCH] trace: Fix preempt_enable_no_resched() abuse

[Peter Zijlstra]

On Tue, Apr 23, 2019 at 09:55:59PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 23, 2019 at 03:41:32PM -0400, Waiman Long wrote:

> > I saw a number of instances of
> > preempt_enable_no_resched() without right next a schedule().
> 
> Look more closely.. and let me know, if true, those are bugs that need
> fixing.
> 
> Argghhh.. BPF...

/me shakes head, Steve...

---
Subject: trace: Fix preempt_enable_no_resched() abuse

Unless the very next line is schedule(), or implies it, one must not use
preempt_enable_no_resched(). It can cause a preemption to go missing and
thereby cause arbitrary delays, breaking the PREEMPT=y invariant.

Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Fixes: 37886f6a9f62 ("ring-buffer: add api to allow a tracer to change clock source")
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/trace/ring_buffer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 41b6f96e5366..4ee8d8aa3d0f 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -762,7 +762,7 @@ u64 ring_buffer_time_stamp(struct ring_buffer *buffer, int cpu)
 
 	preempt_disable_notrace();
 	time = rb_time_stamp(buffer);
-	preempt_enable_no_resched_notrace();
+	preempt_enable_notrace();
 
 	return time;
 }