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

[Gaurav Kohli]

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")

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:

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