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SubjectRe: [RESEND PATCH v2] devres: Really align data field to unsigned long long
On Mon, Jul 09, 2018 at 07:44:44AM +0300, Alexey Brodkin wrote:
> 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.

So is this something you hit today? If not, why is this needed for
stable kernels?

> That's because even on CPUs capable of non-aligned data access LL/SC
> instructions require strict alignment.

Are you going to hit this code with all types of structures? What
happens when you do have an unaligned access?

>
> Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

You didn't cc: this address :(

> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: stable@vger.kernel.org
> ---
>
> Changes v1 -> v2:
>
> * Reworded commit message
> * Inserted comment right in source [Thomas]
>
> drivers/base/devres.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/base/devres.c b/drivers/base/devres.c
> index f98a097e73f2..466fa59c866a 100644
> --- 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.
> + */
> + unsigned long long data[] __aligned(sizeof(unsigned long long));
> };

Does this change the padding today for any other arches? If so, does
the increased memory usage cause any noticable issues?

thanks,

greg k-h

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