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

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Date:   Mon, 9 Jul 2018 14:33:26 +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>,
        "greg@...ah.com" <greg@...ah.com>,
        "will.deacon@....com" <will.deacon@....com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "David.Laight@...LAB.COM" <David.Laight@...LAB.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 16:10 +0200, Peter Zijlstra wrote:
> On Mon, Jul 09, 2018 at 04:07:17PM +0200, Peter Zijlstra wrote:
> > On Mon, Jul 09, 2018 at 04:45:50PM +0300, Alexey Brodkin wrote:
> > > --- a/drivers/base/devres.c
> > > +++ b/drivers/base/devres.c
> > > @@ -24,8 +24,12 @@ struct devres_node {
> > >  
> > >  struct devres {
> > >  	struct devres_node		node;
> > > -	/* -- 3 pointers */
> > > -	unsigned long long		data[];	/* guarantee ull alignment */
> > > +	/*
> > > +	 * data[] must be 64 bit aligned even on 32 bit architectures
> > > +	 * because it might be accessed by instructions that require
> > > +	 * aligned memory arguments such as atomic64_t.
> > > +	 */
> > > +	u8 __aligned(8)			data[];
> > >  };
> > 
> > Seeing that this ends up in a semi generic allocation thing, I don't
> > feel this should be different from ARCH_KMALLOC_MINALIGN.
> 
> 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.

-Alexey