[PATCH] arm64: spinlocks: Fix write starvation with rwlock

On Fri, Sep 08, 2017 at 10:59:59AM -0500, Jeremy Linton wrote:

This patch changes the rwlock behavior so that the writer
unconditionally flags the lock structure (given that its
not already flagged by another writer). This blocks further
readers from acquiring the lock. Once all the readers have
drained, the writer that successfully flagged the lock can
progress.

Recursive readers from IRQ context _should_ be allowed through,
otherwise you'll have deadlocks on tasklist_lock.