linux-kernel - Re: [RESEND PATCH v2] devres: Really align data field to unsigned long long

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Date:   Mon, 9 Jul 2018 10:23:58 +0000
From:   Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:     "David.Laight@...LAB.COM" <David.Laight@...LAB.COM>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "greg@...ah.com" <greg@...ah.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [RESEND PATCH v2] devres: Really align data field to unsigned
 long long

Hi David,

On Mon, 2018-07-09 at 10:18 +0000, David Laight wrote:
> From: Alexey Brodkin
> > Sent: 09 July 2018 11:00
> 
> ...
> > That's a good idea indeed but it doesn't solve the problem with
> > struct devres_node. Consider the following snippet:
> > -------------------------------->8-------------------------------
> > 	struct mystruct {
> > 		atomic64_t myvar;
> > 	}
> > 
> > 	struct mystruct *p;
> > 	p = devm_kzalloc(dev, sizeof(*p), GFP_KERNEL);
> > -------------------------------->8-------------------------------
> > 
> > Here myvar address will match address of "data" member of struct devres_node.
> > So if "data" is has offset of 12 bytes from the beginning of a page then
> > myvar won't be 64-bit aligned regardless of myvar's attribute, right?
> 
> ...
> > > > -	unsigned long long		data[];	/* guarantee ull alignment */
> 
> Ahh, that line should be:
> 	u8 data[] __aligned(8); /* Guarantee 64bit alignment */

And that pretty much what I suggested in my initial patch :)

For the record x86 has exactly the same atomic64_t as you suggested,
see https://elixir.bootlin.com/linux/latest/source/arch/x86/include/asm/atomic64_32.h#L13:
---------------------->8------------------
typedef struct {
	u64 __aligned(8) counter;
} atomic64_t;
---------------------->8------------------

-Alexey