rwlock: add per-cpu reader-writer lock infrastructure

rwlock: add per-cpu reader-writer lock infrastructure

Per-cpu read-write locks allow for the fast path read case to have
low overhead by only setting/clearing a per-cpu variable for using
the read lock. The per-cpu read fast path also avoids locked
compare swap operations which can be particularly slow on coherent
multi-socket systems, particularly if there is heavy usage of the
read lock itself.

The per-cpu reader-writer lock uses a local variable to control
the read lock fast path. This allows a writer to disable the fast
path and ensures the readers switch to using the underlying
read-write lock implementation instead of the per-cpu variable.

Once the writer has taken the write lock and disabled the fast path,
it must poll the per-cpu variable for all CPU's which have entered
the critical section for the specific read-write lock the writer is
attempting to take. This design allows for a single per-cpu variable
to be used for read/write locks belonging to seperate data structures.
If a two or more different per-cpu read lock(s) are taken
simultaneously then the per-cpu data structure is not used and the
implementation takes the read lock of the underlying read-write lock,
this behaviour is equivalent to the slow path in terms of performance.
The per-cpu rwlock is not recursion safe for taking the per-cpu
read lock because there is no recursion count variable, this is
functionally equivalent to standard spin locks.

Slow path readers which are unblocked, set the per-cpu variable and
drop the read lock. This simplifies the implementation and allows
for fairness in the underlying read-write lock to be taken
advantage of.

There is more overhead on the per-cpu write lock path due to checking
each CPUs fast path per-cpu variable but this overhead is likely be
hidden by the required delay of waiting for readers to exit the
critical section. The loop is optimised to only iterate over
the per-cpu data of active readers of the rwlock. The cpumask_t for
tracking the active readers is stored in a single per-cpu data
location and thus the write lock is not pre-emption safe. Therefore
the per-cpu write lock can only be used with interrupts disabled.

Signed-off-by: Malcolm Crossley <malcolm.crossley@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>