Re: [PATCH] trace: Fix race in trace_open and buffer resize call

[Steven Rostedt]

On Tue, 15 Sep 2020 22:53:32 +0530
Gaurav Kohli <gkohli@xxxxxxxxxxxxxx> wrote:

> On 9/15/2020 6:53 PM, Steven Rostedt wrote:
> > On Tue, 15 Sep 2020 10:38:03 +0530
> > Gaurav Kohli <gkohli@xxxxxxxxxxxxxx> wrote:
> >   
> >>  
> >>   >>> +void ring_buffer_mutex_release(struct trace_buffer *buffer)
> >>   >>> +{
> >>   >>> +    mutex_unlock(&buffer->mutex);
> >>   >>> +}
> >>   >>> +EXPORT_SYMBOL_GPL(ring_buffer_mutex_release);  
> >>   >
> >>   > I really do not like to export these.
> >>   >  
> >>
> >> Actually available reader lock is not helping
> >> here(&cpu_buffer->reader_lock), So i took ring buffer mutex lock to
> >> resolve this(this came on 4.19/5.4), in latest tip it is trace buffer
> >> lock. Due to this i have exported api.  
> > 
> > I'm saying, why don't you take the buffer->mutex in the
> > ring_buffer_reset_online_cpus() function? And remove all the protection in
> > tracing_reset_online_cpus()?  
> 
> Yes, got your point. then we can avoid export. Actually we are seeing 
> issue in older kernel like 4.19/4.14/5.4 and there below patch was not 
> present in stable branches:
> 
> ommit b23d7a5f4a07 ("ring-buffer: speed up buffer resets by
>  > avoiding synchronize_rcu for each CPU")  

If you mark this patch for stable, you can add:

Depends-on: b23d7a5f4a07 ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU")  

> 
> Actually i have also thought to take mutex lock in ring_buffer_reset_cpu
> while doing individual cpu reset, but this could cause another problem:

Hmm, I think we should also take the buffer lock in the reset_cpu() call
too, and modify tracing_reset_cpu() the same way.

> 
> Different cpu buffer may have different state, so i have taken lock in 
> tracing_reset_online_cpus.

Why would different states be an issue in synchronizing?

-- Steve

> >
> > void tracing_reset_online_cpus(struct array_buffer *buf)
> > {
> > 	struct trace_buffer *buffer = buf->buffer;
> > 
> > 	if (!buffer)
> > 		return;
> > 
> > 	buf->time_start = buffer_ftrace_now(buf, buf->cpu);
> > 
> > 	ring_buffer_reset_online_cpus(buffer);
> > }
> > 
> > The reset_online_cpus() is already doing the synchronization, we don't need
> > to do it twice.
> > 
> > I believe commit b23d7a5f4a07 ("ring-buffer: speed up buffer resets by
> > avoiding synchronize_rcu for each CPU") made the synchronization in
> > tracing_reset_online_cpus() obsolete.
> > 
> > -- Steve
> >   
> 
> Yes, with above patch no need to take lock in tracing_reset_online_cpus.