linux-kernel - Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

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Date:   Fri, 16 Aug 2019 17:48:59 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     linux-kernel@...r.kernel.org,
        Joel Fernandes <joel@...lfernandes.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>
Subject: Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

On 16/08/2019 17:25, Steven Rostedt wrote:
>> Also, write and read to/from those variables should be done with
>> WRITE_ONCE() and READ_ONCE(), given that those are read within tracing
>> probes without holding the sched_register_mutex.
>>
> 
> I understand the READ_ONCE() but is the WRITE_ONCE() truly necessary?
> It's done while holding the mutex. It's not that critical of a path,
> and makes the code look ugly.
> 

I seem to recall something like locking primitives don't protect you from
store tearing / invented stores, so if you can have concurrent readers
using READ_ONCE(), there should be a WRITE_ONCE() on the writer side, even
if it's done in a critical section.