linux-kernel - RE: [PATCH 09/17] m68k: Implement xor_unlock_is_negative

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Message-ID: <c61a58a1f5a34f2b96c6043840635197@AcuMS.aculab.com>
Date:   Tue, 19 Sep 2023 15:22:25 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Matthew Wilcox' <willy@...radead.org>
CC:     Greg Ungerer <gregungerer@...tnet.com.au>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        Nicholas Piggin <npiggin@...il.com>
Subject: RE: [PATCH 09/17] m68k: Implement xor_unlock_is_negative_byte

From: Matthew Wilcox <willy@...radead.org>
> Sent: 19 September 2023 16:15
> 
> On Tue, Sep 19, 2023 at 02:35:25PM +0000, David Laight wrote:
> > From: Matthew Wilcox <willy@...radead.org>
> > > Sent: 19 September 2023 15:26
> > >
> > > On Tue, Sep 19, 2023 at 01:23:08PM +0000, David Laight wrote:
> > > > > Well, that sucks.  What do you suggest for Coldfire?
> > > >
> > > > Can you just do a 32bit xor ?
> > > > Unless you've got smp m68k I'd presume it is ok?
> > > > (And assuming you aren't falling off a page.)
> > >
> > > Patch welcome.
...
> I have a 68020 book; what I don't have is a Coldfire manual.
> 
> Anyway, that's not the brief.  We're looking to (eg) clear bit 0
> and test whether bit 7 was set.  So it's the sign bit of the byte,
> not the sign bit of the int.

Use the address of the byte as an int and xor with 1u<<24.
The xor will do a rmw on the three bytes following, but I
doubt that matters.

	David

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