linux-kernel - Re: [PATCH] events/uprobes: move smp_read_barrier

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Message-ID: <20160610143959.GA13453@redhat.com>
Date:	Fri, 10 Jun 2016 16:39:59 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrea Parri <parri.andrea@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] events/uprobes: move smp_read_barrier_depends() where
 needed

On 06/10, Oleg Nesterov wrote:
>
> On 06/10, Andrea Parri wrote:
> >
> > More simply/clean, as you said, maybe; one advantage of keeping
> > the "raw" smp_read_barrier_depends() in get_trampoline_vaddr() is
> > that we can avoid it when area is NULL;
>
> Do you really think it makes sense to optimize out read_barrier_depends here?
>
> It can only be NULL in handle_swbp(), and in this case we are going to do a
> lot of work, and in particular install this xol vma,

Not to mention that alpha doesn't support uprobes, so this all is currently
cosmetic.

> > a similar solution is adopt-
> > ed in kernel/task_work.c:task_work_cancel().
>
> Heh ;) this code was written before we had lockless_dereference(). And I do
> remember I thought that we need such a helper when read_barrier_depends()
> was added.

Plus this code still use ACCESS_ONCE for the same reason. I'll send a simple
patch, it should not conflict with "Update spin_unlock_wait users" from Peter.

Oleg.