vm_page_xbusy_claim(): Use atomics to update busy lock state.

[markj]

Summary

vm_page_xbusy_claim() could clobber the waiter bit. For its original
use, kernel memory pages, this was not a problem since nothing would
ever block on the busy lock for such pages. r363607 introduced a new
use where this could in principle be a problem.

Fix the problem by using atomic_cmpset. Since this macro is defined
only for INVARIANTS kernels, the extra overhead doesn't seem
prohibitive.

Reported by: vangyzen

[kib]

sys/vm/vm_page.h

775

I think you want to use _busy_lock as a name. If user code (in kernel) defines a busy_lock macro, we have conflict. The _[a-z0-9] namespace is reserved by std for file-local impl symbols.

[markj]

Put the local var in the file-local namespace.

[vangyzen]

sys/vm/vm_page.h

780

I was expecting fcmpset, but this works, too.

[markj]

sys/vm/vm_page.h

780

I initially wrote it with fcmpset but I find this is a bit clearer. I don't see any real reason to prefer one over the other.