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

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Date:   Mon, 9 Jul 2018 09:16:38 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Alexey Brodkin' <Alexey.Brodkin@...opsys.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Greg KH <greg@...ah.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "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 05:45
> Depending on ABI "long long" type of a particular 32-bit CPU
> might be aligned by either word (32-bits) or double word (64-bits).
> Make sure "data" is really 64-bit aligned for any 32-bit CPU.
> 
> At least for 32-bit ARC cores ABI requires "long long" types
> to be aligned by normal 32-bit word. This makes "data" field aligned to
> 12 bytes. Which is still OK as long as we use 32-bit data only.
> 
> But once we want to use native atomic64_t type (i.e. when we use special
> instructions LLOCKD/SCONDD for accessing 64-bit data) we easily hit
> misaligned access exception.

Shouldn't there be a typedef for the actual type.
Perhaps it is even atomic64_t ?
And have the __aligned(8) applied to that typedef ??

> That's because even on CPUs capable of non-aligned data access LL/SC
> instructions require strict alignment.
...
> --- 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 */
> +	/*
> +	 * Depending on ABI "long long" type of a particular 32-bit CPU
> +	 * might be aligned by either word (32-bits) or double word (64-bits).
> +	 * Make sure "data" is really 64-bit aligned for any 32-bit CPU.

Just:
	/* data[] must be 64 bit aligned even on 32 bit architectures
	 * because it might be accessed by instructions that require
	 * aligned memory arguments.

> +	 */
> +	unsigned long long		data[] __aligned(sizeof(unsigned long long));

One day assuming that 'unsigned long long' is exactly 64 bits will bite.
This probably ought to be u64 (or similar).
(If not atomic64_t)

	David