linux-kernel - Re: [PATCH] ARC: Explicitly set ARCH_SLAB

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Date:   Thu, 14 Feb 2019 11:28:15 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexey Brodkin <alexey.brodkin@...opsys.com>
Cc:     Vineet Gupta <vineet.gupta1@...opsys.com>,
        David Laight <David.Laight@...LAB.COM>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        Arnd Bergmann <arnd.bergmann@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] ARC: Explicitly set ARCH_SLAB_MINALIGN = 8

On Thu, Feb 14, 2019 at 08:50:43AM +0000, Alexey Brodkin wrote:

> But that's pretty much the same for other 32-bit arches that have 64-bit atomics
> like ARM etc. From what I may see from ARM's documentation for LDREXD/SRREXD they
> require double-word alignment of data as well.
> 
> That said if for some reason atomic64_t variable is unaligned execution on
> any (or at least most) 32-bit architectures will lead to run-time failure,
> i.e. we'll know about it and this will be fixed.

On x86_32 we have cmpxchg8b that 'likes' 8b alignment, our atomic64_32
implementation has the explicit alignment in, however there
__alignof__(unsigned long long) is actually 8, so it all works.

Even though the hardware (obviously) never really requires alignment,
even for atomic ops (although that's coming, yay!).