linux-kernel - Re: include/net/sock.h:2100:16: sparse: sparse: cast to non-scalar

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Message-ID: <20260112193019.GK3634291@ZenIV>
Date: Mon, 12 Jan 2026 19:30:19 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Peter Zijlstra <peterz@...radead.org>
Cc: kernel test robot <lkp@...el.com>, Eric Dumazet <edumazet@...gle.com>,
	oe-kbuild-all@...ts.linux.dev, linux-kernel@...r.kernel.org,
	Jakub Kicinski <kuba@...nel.org>,
	Maciej Żenczykowski <maze@...gle.com>,
	Will Deacon <will@...nel.org>,
	"Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: include/net/sock.h:2100:16: sparse: sparse: cast to non-scalar

On Mon, Jan 12, 2026 at 01:32:24PM +0100, Peter Zijlstra wrote:
> On Sat, Jan 10, 2026 at 10:35:48PM +0000, Al Viro wrote:
> 
> > #define __READ_ONCE(x)                                                  \
> > ({                                                                      \
> >         __unqual_scalar_typeof(x) __x =                                 \
> > 		(*(volatile typeof(__x) *)(&(x)));                      \
> > 	mb();                                                           \
> > 	(typeof(x))__x;                                                 \
> > })
> > combined with
> > typedef struct {
> >         uid_t val;
> > } kuid_t;
> > 
> > IOW, it complains about a cast from structure to itself, which is fair
> > enough - C is pretty clear about not allowing any typecasts to or from
> > non-scalar types, tautological or not.
> > 
> > Why do we even need that cast?  Seeing that generic __READ_ONCE() is
> > #define __READ_ONCE(x)  (*(const volatile __unqual_scalar_typeof(x) *)&(x))
> > the cast added on alpha seems to be pointless.
> 
> The problem was things like test_bit() that take a volatile argument,
> doing READ_ONCE() on them would instantiate a volatile temporary and GCC
> would end up generating shit code.
> 
> The __unqual_scalar_typeof() was the result of trying to remove CV
> qualifiers from a type.

Sure, but what does that have to do with the final cast in there?  Note
that generic __READ_ONCE() ends up with __unqual_scalar_typeof(x) for the
type of result and it seems to be working fine.

Why would the same type of result be a problem for alpha?  Sure, we need
a barrier there, so it can't be literal copy of generic __READ_ONCE(),
but what's the problem with using

#define __READ_ONCE(x)							\
({									\
	__unqual_scalar_typeof(x) __x =					\
 		(*(volatile typeof(__x) *)(&(x)));			\
	mb();								\
	__x;								\
})

there?  What could possibly need those qualifiers added back to the result?
It is, after all, an r-value, so...

IDGI...