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SubjectRe: [RESEND PATCH v2] devres: Really align data field to unsigned long long
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Hi David,

On Mon, 2018-07-09 at 09:16 +0000, David Laight wrote:
> 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 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?


> > 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).

I agree. Initially I wanted to keep as few changes as possible but
IMHO switching to more predictable data type makes sense.

-Alexey
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