linux-kernel - Re: [PATCH 1/8] sparc32: make __cmpxchg

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Date: Wed, 3 Apr 2024 20:09:21 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: linux-kernel@...r.kernel.org, kernel-team@...a.com,
	"David S. Miller" <davem@...emloft.net>,
	Andreas Larsson <andreas@...sler.com>,
	Palmer Dabbelt <palmer@...osinc.com>, Arnd Bergmann <arnd@...db.de>,
	Marco Elver <elver@...gle.com>
Subject: Re: [PATCH 1/8] sparc32: make __cmpxchg_u32() return u32

On Wed, Apr 03, 2024 at 11:20:53PM +0100, Al Viro wrote:
> On Tue, Apr 02, 2024 at 01:03:13PM -0700, Paul E. McKenney wrote:
> > On Tue, Apr 02, 2024 at 12:28:28AM -0400, Al Viro wrote:
> > > Conversion between u32 and unsigned long is tautological there,
> > > and the only use of return value is to return it from
> > > __cmpxchg() (which return unsigned long).
> > > 
> > > Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
> > 
> > I have pulled these in as replacements for my patches in the meantime.
> > 
> > Thank you!
> 
> FWIW, updated branch force-pushed; the difference is that __cmpxchg()
> on sparc32 went
> -       switch (size) {
> -       case 1:
> -               return __cmpxchg_u8((u8 *)ptr, (u8)old, (u8)new_);
> -       case 2:
> -               return __cmpxchg_u16((u16 *)ptr, (u16)old, (u16)new_);
> -       case 4:
> -               return __cmpxchg_u32((u32 *)ptr, (u32)old, (u32)new_);
> -       default:
> -               __cmpxchg_called_with_bad_pointer();
> -               break;
> -       }
> -       return old;
> +       return
> +               size == 1 ? __cmpxchg_u8(ptr, old, new_) :
> +               size == 2 ? __cmpxchg_u16(ptr, old, new_) :
> +               size == 4 ? __cmpxchg_u32(ptr, old, new_) :
> +                       (__cmpxchg_called_with_bad_pointer(), old);
> 
> (and similar for parisc).  Rationale: sparse does generate constant
> truncation warnings in unreachable statements, but not in never-evaluated
> subexpressions.  Alternative would be what parisc used to do in mainline:
> 	case 1: return __cmpxchg_u8((u8 *)ptr, old & 0xff, new_ & 0xff);
> and we'd need the same in 16bit case (both on parisc and sparc32).
> Explicit (and rather mysterious) & 0xff for passing unsigned long to
> a function that takes u8 was there to tell sparse that e.g.
> cmpxchg(&int_var, 0, 0x12345678) was *not* trying to feed
> 0x12345678 to a __cmpxchg_u8(), which would quietly truncate it had
> it ever been reached.  Use of conditional expression avoids that
> without having to play with explicit (and utterly pointless from
> C point of view) masking.  IMO it's better that way, not to mention
> being more concise than use of switch.

Cute!  I replaced the old versions of your patches with this series.

However, I was too lazy to apply this transformation to the other
cmpxchg() implementations.  ;-)

							Thanx, Paul