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

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Message-ID: <e98bd4ab-ea20-0b42-bda9-eae8a6030c8f@synopsys.com>
Date:   Mon, 9 Jul 2018 11:27:31 -0700
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        "David.Laight@...LAB.COM" <David.Laight@...LAB.COM>
CC:     "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "greg@...ah.com" <greg@...ah.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
Subject: Re: [RESEND PATCH v2] devres: Really align data field to unsigned
 long long

On 07/09/2018 03:23 AM, Alexey Brodkin wrote:
> 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------------------

And so does the ARC version since when the atomic64_t support was added by commit
ce6365270ecd

Also, you should consider using the pre-canned type aligned_u64.

typedef struct {
       aligned_u64 counter;
       ^^^^^^^^^^^
} atomic64_t;