Re: [patch V2 0/7] fs: Substitute bit-spinlocks for PREEMPT_RT and debugging

[Christoph Hellwig]

On Thu, Aug 08, 2019 at 09:02:47AM +0200, Thomas Gleixner wrote:
> > >   mm/slub.c:      bit_spin_lock(PG_locked, &page->flags);
> > 
> > One caller ouf of a gazillion that spins on the page lock instead of
> > sleepign on it like everyone else.  That should not have passed your
> > smell test to start with :)
> 
> I surely stared at it, but that cannot sleep. It's in the middle of a
> preempt and interrupt disabled region and used on architectures which do
> not support CMPXCHG_DOUBLE and ALIGNED_STRUCT_PAGE ...

I know.  But the problem here is that normally PG_locked is used together 
with wait_on_page_bit_*, but this one instances uses the bit spinlock
helpers.  This is the equivalent of calling spin_lock on a struct mutex
rather than having a mutex_lock_spin helper for this case.  Does SLUB
work on -rt at all?