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Date:   Tue, 11 Aug 2020 09:49:25 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Jann Horn <jannh@google.com>, Jens Axboe <axboe@kernel.dk>,
        io-uring <io-uring@vger.kernel.org>,
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Subject: Re: [PATCH 2/2] io_uring: use TWA_SIGNAL for task_work if the task
 isn't running
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On Tue, Aug 11, 2020 at 09:26:37AM +0200, Oleg Nesterov wrote:
> On 08/11, Oleg Nesterov wrote:
> >
> > On 08/11, Peter Zijlstra wrote:
> > >
> > > On Tue, Aug 11, 2020 at 08:45:16AM +0200, Oleg Nesterov wrote:
> > > >
> > > > ->jobctl is always modified with ->siglock held, do we really need
> > > > WRITE_ONCE() ?
> > >
> > > In theory, yes. The compiler doesn't know about locks, it can tear
> > > writes whenever it feels like it.
> >
> > Yes, but why does this matter? Could you spell please?
> 
> Do you mean that compiler can temporary set/clear JOBCTL_TASK_WORK
> when it sets/clears another bit?

Possibly, afaict the compiler is allowed to 'spill' intermediate state
into the variable. If any intermediate state has the bit clear,...