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Date:   Mon, 9 Jul 2018 16:10:56 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Cc:     linux-kernel@vger.kernel.org, linux-snps-arc@lists.infradead.org,
        linux-arch@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        David Laight <David.Laight@ACULAB.COM>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <will.deacon@arm.com>, Greg KH <greg@kroah.com>,
        stable@vger.kernel.org
Subject: Re: [PATCH v3] devres: Explicitly align datai[] to 64-bit
Message-ID: <20180709141056.GR2512@hirez.programming.kicks-ass.net>
References: <20180709134550.29541-1-abrodkin@synopsys.com>
 <20180709140717.GR2476@hirez.programming.kicks-ass.net>
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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.