Re: [RFC PATCH 04/16] x86/split_lock: Use non locked bit set instruction in set_cpu_cap

[Peter Zijlstra <peterz@infradead.org>]

On Sun, May 27, 2018 at 08:45:53AM -0700, Fenghua Yu wrote:
> set_bit() called by set_cpu_cap() is a locked bit set instruction for
> atomic operation.
> 
> Since the c->x86_capability can span two cache lines depending on kernel
> configuration and building evnironment, the locked bit set instruction may
> cause #AC exception when #AC exception for split lock is enabled.

That doesn't make sense. Sure the bitmap may be longer, but depending on
if the argument is an immediate or not we either use a byte instruction
(which can never cross a cacheline boundary) or a 'word' aligned BTS.
And the bitmap really _should_ be 'unsigned long' aligned.

If it is not aligned, fix that too.

/me looks at cpuinfo_x86 and finds x86_capability is in fact a __u32
array.. see that's broken and needs fixing first.