linux-kernel - Re: [PATCH cmpxchg 13/14] xtensa: Emulate one-byte cmpxchg

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Date: Fri, 19 Apr 2024 10:02:47 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Yujie Liu <yujie.liu@...el.com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, linux-arch@...r.kernel.org, 
	linux-kernel@...r.kernel.org, elver@...gle.com, akpm@...ux-foundation.org, 
	tglx@...utronix.de, peterz@...radead.org, dianders@...omium.org, 
	pmladek@...e.com, torvalds@...ux-foundation.org, 
	Arnd Bergmann <arnd@...db.de>, Andi Shyti <andi.shyti@...ux.intel.com>
Subject: Re: [PATCH cmpxchg 13/14] xtensa: Emulate one-byte cmpxchg

On Fri, Apr 19, 2024 at 7:14 AM Yujie Liu <yujie.liu@...el.com> wrote:
> On Thu, Apr 18, 2024 at 04:21:46PM -0700, Paul E. McKenney wrote:
> > On Thu, Apr 18, 2024 at 10:06:21AM +0200, Geert Uytterhoeven wrote:
> > > On Mon, Apr 8, 2024 at 7:49 PM Paul E. McKenney <paulmck@...nel.org> wrote:
> > > > Use the new cmpxchg_emu_u8() to emulate one-byte cmpxchg() on xtensa.
> > > >
> > > > [ paulmck: Apply kernel test robot feedback. ]
> > > > [ paulmck: Drop two-byte support per Arnd Bergmann feedback. ]
> > > >
> > > > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > >
> > > Thanks for your patch!
> > >
> > > > --- a/arch/xtensa/include/asm/cmpxchg.h
> > > > +++ b/arch/xtensa/include/asm/cmpxchg.h
> > > > @@ -74,6 +75,7 @@ static __inline__ unsigned long
> > > >  __cmpxchg(volatile void *ptr, unsigned long old, unsigned long new, int size)
> > > >  {
> > > >         switch (size) {
> > > > +       case 1:  return cmpxchg_emu_u8((volatile u8 *)ptr, old, new);
> > >
> > > The cast is not needed.
> >
> > In both cases, kernel test robot yelled at me when it was not present.
> >
> > Happy to resubmit without it, though, if that is a yell that I should
> > have ignored.
>
> FYI, kernel test robot did yell some reports on various architectures such as:
>
> [1] https://lore.kernel.org/oe-kbuild-all/202403292321.T55etywH-lkp@intelcom/
> [2] https://lore.kernel.org/oe-kbuild-all/202404040526.GVzaL2io-lkp@intelcom/
> [3] https://lore.kernel.org/oe-kbuild-all/202404022106.mYwpypit-lkp@intelcom/
>
> In brief, there were mainly three types of issues:
>
> * The cmpxchg-emu.h header is missing
> * The parameters of cmpxchg_emu_u8 need to be cast to corresponding types
> * The return value of cmpxchg_emu_u8 needs to be cast to the "ret" type
>
> As for this specific case of xtensa arch, the compiler doesn't warn
> regardless of whether there is an explicit cast for "ptr" or not.
> The "ptr" being passed in is "void *", and it seems that a "void *"
> pointer can be automatically cast to any other type of pointer, so it
> is not necessary to have an explicit cast of "u8 *".
>
> As for the implementations of other architectures that don't pass the
> "ptr" as "void *" (such as a macro implementation), the explicit cast to
> "u8 *" may still be required.

Exactly.  On sh and xtensa, the original pointer is of type
"volatile void *", so no cast is needed.
On E.g. arc, the original pointer is of type "volatile __typeof__(ptr) _p_",
which is not always compatible with "volatile u8 *".

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68korg

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds