linux-kernel - Re: [PATCH 1/2] tracing: Simplify and fix "buffered event" synchronization

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Message-ID: <490c77e9-e3d4-4499-8471-128804fb2e7a@suse.com>
Date:   Tue, 5 Dec 2023 17:16:13 +0100
From:   Petr Pavlu <petr.pavlu@...e.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     mhiramat@...nel.org, mathieu.desnoyers@...icios.com,
        zhengyejian1@...wei.com, linux-trace-kernel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] tracing: Simplify and fix "buffered event"
 synchronization

On 12/1/23 15:46, Steven Rostedt wrote:
> On Fri, 1 Dec 2023 15:17:35 +0100
> Petr Pavlu <petr.pavlu@...e.com> wrote:
> 
>> Ok, keeping the current approach, my plan for v2 is to prepare the
>> following patches:
>>
>> [...]
>> * Fix the potential race between trace_buffered_event_enable() and
>>   trace_event_buffer_lock_reserve() where the latter might already see
>>   a valid trace_buffered_event pointer but not all initialization yet.
>>
>>   I think this might be actually best to address by using the same
>>   maintenance exclusion as is implemented in
>>   trace_buffered_event_disable(). It would make both maintenance
>>   operations consistent but for the cost of making the enable operation
>>   somewhat slower.
> 
> I wouldn't do them the same just to make them consistent. I think the
> smp_wmb() is sufficient. Don't you think?

Looking at this again, I think it is actually a non-issue. Function
trace_buffered_event_enable() only writes the header part of
ring_buffer_event but that is never written nor read by the actual users
which obtain the buffer from trace_event_buffer_lock_reserve().

No change is then needed, it is left out in v2 of the series.

-- Petr