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

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Message-ID: <20190214103140.GG32494@hirez.programming.kicks-ass.net>
Date:   Thu, 14 Feb 2019 11:31:40 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vineet Gupta <vineet.gupta1@...opsys.com>
Cc:     David Laight <David.Laight@...LAB.COM>,
        Alexey Brodkin <alexey.brodkin@...opsys.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 Wed, Feb 13, 2019 at 03:23:36PM -0800, Vineet Gupta wrote:
> On 2/13/19 4:56 AM, Peter Zijlstra wrote:
> >
> > Personally I think u64 and company should already force natural
> > alignment; but alas.
> 
> But there is an ISA/ABI angle here too. e.g. On 32-bit ARC, LDD (load double) is
> allowed to take a 32-bit aligned address to load a register pair. Thus all u64
> need not be 64-bit aligned (unless attribute aligned 8 etc) hence the relaxation
> in ABI (alignment of long long is 4). You could certainly argue that we end up
> undoing some of it anyways by defining things like ARCH_KMALLOC_MINALIGN to 8, but
> still...

So what happens if the data is then split across two cachelines; will a
STD vs LDD still be single-copy-atomic? I don't _think_ we rely on that
for > sizeof(unsigned long), with the obvious exception of atomic64_t,
but yuck...

So even though it is allowed by the chip; does it really make sense to
use this?