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

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Message-ID: <bfbf4d9ae5674d5dbe8c509abf5b0f84@AcuMS.aculab.com>
Date:   Tue, 19 Sep 2023 15:57:36 +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
> Sent: 19 September 2023 16:47
> 
> On Tue, Sep 19, 2023 at 03:22:25PM +0000, David Laight wrote:
> > > 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.
> 
> Bet you a shiny penny that Coldfire takes an unaligned access trap ...

and then the 'firmware' silently fixed it up for you a few 1000
clocks later...

> and besides, this is done on _every_ call to unlock_page().  That might
> cross not only a cacheline boundary but also a page boundary.  I cannot
> believe that would be a high-performing solution.  It might be just fine
> on m68000 but I bet even by the 030 it's lower performing.

I do remember managing to use 'cas2' to add an item to a linked list.
But it is so painful so setup it was better just to disable interrupts.
For non-smp that is almost certainly ok.
(Unless the instructions are slow because of synchronisation.)
Otherwise you need to use 'cas' on the aligned word.
Assuming coldfire even has cas.

	David

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