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

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Date:   Fri, 6 Jul 2018 21:10:37 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>
cc:     linux-kernel@...r.kernel.org, linux-snps-arc@...ts.infradead.org,
        linux-arch@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] devres: Really align data field to unsigned long long

On Fri, 6 Jul 2018, Alexey Brodkin wrote:

> It looks like on most of architectures "data" member of devres struture
> gets aligned to 8-byte "unsigned long long" boundary as one may expect:
> if we don't explicitly pack a structure then natural alignment
> (which matches each member data type) is used.
> 
> But at least on 32-bit ARC architecture ABI requires "long long" types
> to be aligned by normal 32-bit word. This makes "data" field aligned to
> 12 bytes. This 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.
> 
> That's because even on CPUs capable of non-aligned data access LL/SC
> instructions require strict alignment.
> 
> Signed-off-by: Alexey Brodkin <abrodkin@...opsys.com>
> Cc: stable@...r.kernel.org
> ---
>  drivers/base/devres.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/base/devres.c b/drivers/base/devres.c
> index f98a097e73f2..35ddc8b66bc9 100644
> --- a/drivers/base/devres.c
> +++ b/drivers/base/devres.c
> @@ -25,7 +25,7 @@ struct devres_node {
>  struct devres {
>  	struct devres_node		node;
>  	/* -- 3 pointers */
> -	unsigned long long		data[];	/* guarantee ull alignment */

Pretty please put a big fat comment here so that the innocent reader does
not start to wonder.

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

Thanks,

	tglx