Re: linux-next: build warnings after merge of the access_once tree

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Mar 26, 2015 at 07:31:12PM +1100, Stephen Rothwell wrote:
> Hi Christian,
> 
> After merging the access_once tree, today's linux-next build (arm
> multi_v7_defconfig) produced lots of this warning:
> 
> In file included from include/linux/linkage.h:4:0,
>                  from include/linux/preempt.h:9,
>                  from include/linux/spinlock.h:50,
>                  from include/linux/lockref.h:17,
>                  from lib/lockref.c:2:
> In function '__read_once_size',
>     inlined from 'lockref_get' at lib/lockref.c:50:2:
> include/linux/compiler.h:216:3: warning: call to 'data_access_exceeds_word_size' declared with attribute warning: data access exceeds word size and won't be atomic
>    data_access_exceeds_word_size();
>    ^
> 
> Introduced by commit 6becd6bd5e89 ("compiler.h: Fix word size check for
> READ/WRITE_ONCE") presumably interacting with commit 4d3199e4ca8e
> ("locking: Remove ACCESS_ONCE() usage") from the tip tree.

Hmm, valid warning though, ARM is a 32bit arch and therefore it will
'have' to load a u64 in two goes, which violates the
ACCESS_ONCE/READ_ONCE 'promise'.

Now ARM can indeed to the cmpxchg64 thing, but I'm not sure what to do
here, I suspect the code is fine, seeing how the cmpxchg64 will fail if
the split loads got it wrong, but I've not overly thought about it.