[PATCHv3 0/2] spinlock: queued read-write locks

[David Vrabel <david.vrabel@citrix.com>]

This series replaces the current read-write lock implementation with
the queued read-write locks from Linux.  These are fair; under
contention both readers and writers will be queued and obtain the lock
in FIFO order (due to the fairness of the internal ticket lock).

The implementation is all in C and thus architecture independent.

Compared to the Linux implementation some of the memory barrier
primitives were changed.  The ARM maintainers may want to give this
area a careful review.

Changes in v3:

- Fix CONFIG_XSM build.

Significant changes in v2:

- atomic_cmpxchg() is now arch-dependent as arm/arm64 already provided
  their own.
- Moved rwlocks into their own file.
- Removed the special casing of in_irq() in the slow read path.

David