The existing per-cpu superblock counter code uses the global superblock
spin lock when we approach ENOSPC for global synchronisation. On larger
machines than this code was originally tested on this can still get
catastrophic spinlock contention due increasing rebalance frequency near
ENOSPC.

By introducing a sleeping lock that is used to serialise balances and
modifications near ENOSPC we prevent contention from needlessly from
wasting the CPU time of potentially hundreds of CPUs.

To reduce the number of balances occuring, we separate the need rebalance
case from the slow allocate case. Now, a counter running dry will trigger
a rebalance during which counters are disabled. Any thread that sees a
disabled counter enters a different path where it waits on the new mutex.
When it gets the new mutex, it checks if the counter is disabled. If the
counter is disabled, then we _know_ that we have to use the global counter
and lock and it is safe to do so immediately. Otherwise, we drop the mutex
and go back to trying the per-cpu counters which we know were re-enabled.

SGI-PV: 952227
SGI-Modid: xfs-linux-melb:xfs-kern:27612a
Signed-off-by: NDavid Chinner <dgc@sgi.com>
Signed-off-by: NLachlan McIlroy <lachlan@sgi.com>
Signed-off-by: NTim Shimmin <tes@sgi.com>