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

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Date: Wed, 3 Apr 2024 23:20:53 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: "Paul E. McKenney" <paulmck@...nel.org>
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 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.