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

[Fenghua Yu]

On Thu, Jun 21, 2018 at 09:55:40PM +0200, Peter Zijlstra wrote:
> 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.

Do you mean x86_capability's type should be changed from __u32 to unsigned
long?

Changing x86_capability's type won't directly fix the split lock in
set_cpu_cap(), right? BTS still may access x86_capability across cache
line no matter x86_capability's type.

Thanks.

-Fenghua