linux-kernel - Re: [PATCH v3] devres: Explicitly align datai[] to 64-bit

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Date:   Thu, 4 Oct 2018 16:19:16 +0000
From:   Alexey Brodkin <alexey.brodkin@...opsys.com>
To:     "peterz@...radead.org" <peterz@...radead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "will.deacon@....com" <will.deacon@....com>,
        "greg@...ah.com" <greg@...ah.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "David.Laight@...LAB.COM" <David.Laight@...LAB.COM>,
        Vineet Gupta <vineet.gupta1@...opsys.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "geert@...ux-m68k.org" <geert@...ux-m68k.org>
Subject: Re: [PATCH v3] devres: Explicitly align datai[] to 64-bit

Hi Peter,

On Mon, 2018-07-09 at 17:53 +0300, Alexey Brodkin wrote:
> Hi Peter,
> 
> On Mon, 2018-07-09 at 16:49 +0200, Peter Zijlstra wrote:
> > On Mon, Jul 09, 2018 at 02:33:26PM +0000, Alexey Brodkin wrote:
> > > > In fact, since alloc_dr() uses kmalloc() to allocate the entire thing,
> > > > it is impossible to guarantee a larger alignment than kmalloc does.
> > > 
> > > Well but 4-bytes [which is critical for atomic64_t] should be much less
> > > than a sane cache line length so above should work.
> > 
> > AFAICT ARCH_KMALLOC_MINALIGN ends up being 4 on x86_32 (it doesn't
> > define ARCH_DMA_MINALIGN and doesn't seem to otherwise override the
> > thing).
> > 
> > So unconditionally setting the alignment of devres::data to 8 seems
> > broken.
> 
> Well but then what other options do we have to fix a problem with
> misaligned access to atomic64_t in drm_gpu_scheduler?

Ping!

Should I send v4 with ARCH_KMALLOC_MINALIGN used for alignment
if explicitly set __aligned__(8) will break x86_32?

-Alexey