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

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Message-ID: <ZQnCiZuMbFnwbEUt@casper.infradead.org>
Date:   Tue, 19 Sep 2023 16:47:21 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Laight <David.Laight@...lab.com>
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

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 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.