linux-kernel - Re: [PATCH] seqlock: fix raw_read_seqcount

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Date:	Mon, 23 May 2016 11:36:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] seqlock: fix raw_read_seqcount_latch()

On Sun, May 22, 2016 at 09:50:40PM +0300, Alexey Dobriyan wrote:
> On Sun, May 22, 2016 at 12:48:27PM +0200, Peter Zijlstra wrote:
> > On Sat, May 21, 2016 at 11:14:49PM +0300, Alexey Dobriyan wrote:
> > > lockless_dereference() is supposed to take pointer not integer.
> > 
> > Urgh :/
> > 
> > Is there any way we can make lockless_dereference() issue a warning if
> > we don't feed it a pointer?
> > 
> > Would something like so work? All pointer types should silently cast to
> > void * while integer (and others) should refuse to.
> 
> This works (and spammy enough in case of seqlock, which is good)
> but not for "unsigned long":
> 
> 	include/linux/percpu-refcount.h:146:36: warning: initialization makes pointer from integer without a cast [-Wint-conversion]
> 	  percpu_ptr = lockless_dereference(ref->percpu_count_ptr);

TJ; would you prefer casting or not using lockless_dereference() here?

> 
> > --- a/include/linux/compiler.h
> > +++ b/include/linux/compiler.h
> > @@ -544,6 +544,7 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
> >   */
> >  #define lockless_dereference(p) \
> >  ({ \
> > + 	__maybe_unused void * _________p2 = p; \
> >  	typeof(p) _________p1 = READ_ONCE(p); \
> >  	smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
> >  	(_________p1); \