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

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Date:   Mon, 9 Jul 2018 10:18:36 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Alexey Brodkin' <Alexey.Brodkin@...opsys.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

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 */

	David