rwsem: use cmpxchg for trying to steal write lock

Using rwsem_atomic_update to try stealing the write lock forced us to
undo the adjustment in the failure path. We can have simpler and faster
code by using cmpxchg instead.

Signed-off-by: Michel Lespinasse <walken@google.com>

Conflicts:

	lib/rwsem.c
rwsem: avoid taking wait_lock in rwsem_down_write_failed

In rwsem_down_write_failed(), if there are active locks after we wake up
(i.e. the lock got stolen from us), skip taking the wait_lock and go back
to sleep immediately.

Signed-off-by: Michel Lespinasse <walken@google.com>
rwsem: skip initial trylock in rwsem_down_write_failed

We can skip the initial trylock in rwsem_down_write_failed() if there are
known active lockers already, thus saving one likely-to-fail cmpxchg.

Signed-off-by: Michel Lespinasse <walken@google.com>
rwsem: simplify __rwsem_do_wake

This is mostly for cleanup value:

- We don't need several gotos to handle the case where the first
  waiter is a writer. Two simple tests will do (and generate very
  similar code).

- In the remainder of the function, we know the first waiter is a reader,
  so we don't have to double check that. We can use do..while loops
  to iterate over the readers to wake (generates slightly better code).

Signed-off-by: Michel Lespinasse <walken@google.com>
rwsem: implement support for write lock stealing on the fastpath

When we decide to wake up readers, we must first grant them as many
read locks as necessary, and then actually wake up all these readers.
But in order to know how many read shares to grant, we must first
count the readers at the head of the queue. This might take a while
if there are many readers, and we want to be protected against a
writer stealing the lock while we're counting. To that end, we grant
the first reader lock before counting how many more readers are queued.

We also require some adjustments to the wake_type semantics.

RWSEM_WAKE_NO_ACTIVE used to mean that we had found the count to
be RWSEM_WAITING_BIAS, in which case the rwsem was known to be free
as nobody could steal it while we hold the wait_lock. This doesn't
make sense once we implement fastpath write lock stealing, so we now
use RWSEM_WAKE_ANY in that case.

Similarly, when rwsem_down_write_failed found that a read lock was active,
it would use RWSEM_WAKE_READ_OWNED which signalled that new readers could
be woken without checking first that the rwsem was available. We can't do
that anymore since the existing readers might release their read locks,
and a writer could steal the lock before we wake up additional readers.
So, we have to use a new RWSEM_WAKE_READERS value to indicate we only want
to wake readers, but we don't currently hold any read lock.

Signed-off-by: Michel Lespinasse <walken@google.com>
rwsem: do not block readers at head of queue if other readers are active

This change fixes a race condition where a reader might determine it
needs to block, but by the time it acquires the wait_lock the rwsem
has active readers and no queued waiters.

In this situation the reader can just in parallel with the existing active
readers; it does not need to block until the active readers complete.

Thanks to Peter Hurley for noticing this possible race.

Signed-off-by: Michel Lespinasse <walken@google.com>