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Subject: Re: [PATCH] trace: Fix race in trace_open and buffer resize call
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     mingo@redhat.com, linux-kernel@vger.kernel.org,
        linux-arm-msm@vger.kernel.org
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From:   Gaurav Kohli <gkohli@codeaurora.org>
Message-ID: <08d6f338-3be3-c5a2-ba4b-0116de9672c2@codeaurora.org>
Date:   Tue, 15 Sep 2020 22:53:32 +0530
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On 9/15/2020 6:53 PM, Steven Rostedt wrote:
> On Tue, 15 Sep 2020 10:38:03 +0530
> Gaurav Kohli <gkohli@codeaurora.org> 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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