linux-kernel - Re: [PATCH 17/19] locking/barriers: Kill lockless

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Date:   Tue, 24 Oct 2017 11:54:28 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Will Deacon <will.deacon@....com>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        mark.rutland@....com, snitzer@...hat.com,
        thor.thayer@...ux.intel.com, viro@...iv.linux.org.uk,
        davem@...emloft.net, shuah@...nel.org, mpe@...erman.id.au,
        tj@...nel.org, torvalds@...ux-foundation.org
Subject: Re: [PATCH 17/19] locking/barriers: Kill lockless_dereference


* Will Deacon <will.deacon@....com> wrote:

> On Tue, Oct 24, 2017 at 11:31:04AM +0200, Ingo Molnar wrote:
> > 
> > * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> > 
> > > From: Will Deacon <will.deacon@....com>
> > > 
> > > lockless_dereference is a nice idea, but its gained little traction in
> > > kernel code since it's introduction three years ago. This is partly
> > 
> > s/its/it
> > s/it's/its
> 
> Crikey, no idea what happened there!

NP, just habitual drive-by typo fixing from me, would have fixed them silently 
when applying the patches, had it not been for the split-up suggestion :-)

> > > because it's a pain to type, but also because using READ_ONCE instead
> > > will work correctly on all architectures apart from Alpha, which is a
> > > fully supported but somewhat niche architecture these days.
> > > 
> > > This patch moves smp_read_barrier_depends() (a NOP on all architectures
> > > other than Alpha) from lockless_dereference into READ_ONCE, converts
> > > the few actual users over to READ_ONCE and then finally removes
> > > lockless_dereference altogether.
> > 
> > Nit: if we refer to smp_read_barrier_depends() with parentheses (which is the nice 
> > thing to do for function-alike symbols), then we should do the same with 
> > READ_ONCE() and lockless_dereference() as well.
> > 
> > Also, could we please split this into three patches:
> > 
> >  #1: Add smp_read_barrier_depends() to READ_ONCE()
> >  #2: Convert all lockless_dereference() users to READ_ONCE()
> >  #3: Remove the now unused lockless_dereference() API
> > 
> > to make it easier to analyze if bisected to, should any problems arise?
> 
> Sure, I'll do that now.

Thanks!

	Ingo