Re: [PATCH] trace: Fix preempt_enable_no_resched() abuse

[Steven Rostedt]

On Tue, 23 Apr 2019 22:03:18 +0200
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> 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...

/me points finger to Frederic ;-)


> 
> ---
> 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")

That commit just moved the buggy code. That tag should be:

Fixes: 2c2d7329d8af ("tracing/ftrace: use preempt_enable_no_resched_notrace in ring_buffer_time_stamp()")

OK, this isn't quite fair to point all the blame on Frederic, because
it did fix a bug. But the real fix for that bug was your fix here:

499d79559ffe4b ("sched/core: More notrace annotations")

-- Steve


> 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;
>  }