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

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Message-ID: <20160610122537.GA7776@redhat.com>
Date:	Fri, 10 Jun 2016 14:26:18 +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 06/09, 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?

Oleg.