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

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Date:	Fri, 10 Jun 2016 15:07:33 +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, Andrea Parri wrote:
>
> On Fri, Jun 10, 2016 at 2:26 PM, Oleg Nesterov <oleg@...hat.com> wrote:
>
> > On 06/10, Oleg Nesterov 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.
> >
> > and perhaps it would be better to eliminate this smp_read_barrier_depends()
> > altogether, we have a lot of helpers. I mean,
> >
> >         static unsigned long get_trampoline_vaddr(void)
> >         {
> >                 struct xol_area *area;
> >
> >                 area =
> > lockless_dereference(current->mm->uprobes_state.xol_area);
> >                 if (area)
> >                         return area->vaddr;
> >                 return -1;
> >         }
> >
> > looks a bit more simple/clean. Note also another smp_read_barrier_depends()
> > in get_xol_area() which can be changed the same way.
> >
> > What do you think?
> >
>
> 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,

> 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.

Oleg.