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From: Oleg Nesterov <oleg@redhat.com>
To: Andrea Parri <parri.andrea@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] events/uprobes: move smp_read_barrier_depends() where needed
Date: Fri, 10 Jun 2016 14:59:04 +0200	[thread overview]
Message-ID: <20160610125904.GA8203@redhat.com> (raw)
In-Reply-To: <CAPZ9YJaxdqpCh9DyD7ubrTEhCL9jEdMzNN6zJHsOPYGN5NNPXg@mail.gmail.com>

On 06/10, Andrea Parri wrote:
>
> On Fri, Jun 10, 2016 at 9:03 AM, Oleg Nesterov <oleg@redhat.com> wrote:
>
> > On 06/09, Andrea Parri wrote:
> > >
> > > There is no need to use the barrier if there is no dereference/
> > > memory access; move it where needed (currently, affecting only
> > > Alpha).
> >
> > OK, although area == NULL is unlikely case,
> >
> > > While touching this, also make the reads _ONCE().
> >
> > Why? both xol_area/vaddr can't change.
> >
>
> This is to ensure current (and future) compiler transformations
> won't optimize out or reorder the reads (do not ask me how ...,
> IAC the volatile casts will prevent this to happen).
>
> AFAICT, xol_area can be "== NULL" and "!= NULL" ...

Yes, but it can't change if it is not NULL. So we do not care if gcc
reads xol_area/vaddr twice or reorderes this with other LOADs.

Oleg.

      parent reply	other threads:[~2016-06-10 12:59 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-06-09 18:18 [PATCH] events/uprobes: move smp_read_barrier_depends() where needed Andrea Parri
2016-06-10  7:03 ` Oleg Nesterov
2016-06-10 12:26   ` Oleg Nesterov
     [not found]     ` <CAPZ9YJY_6mB797YRG4=byTbz6WTH91GZjba9zDQhZ+J0C-khSg@mail.gmail.com>
2016-06-10 13:07       ` Oleg Nesterov
2016-06-10 14:39         ` Oleg Nesterov
     [not found]           ` <CAPZ9YJYPynX2vGgFarhiyUN0GF--h6r2kbhJ4pRt2NMY1qCODg@mail.gmail.com>
2016-06-10 19:36             ` Oleg Nesterov
     [not found]   ` <CAPZ9YJaxdqpCh9DyD7ubrTEhCL9jEdMzNN6zJHsOPYGN5NNPXg@mail.gmail.com>
2016-06-10 12:59     ` Oleg Nesterov [this message]

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