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

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Message-ID: <20160522185040.GA23664@p183.telecom.by>
Date:	Sun, 22 May 2016 21:50:40 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] seqlock: fix raw_read_seqcount_latch()

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);

> --- 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); \