An RT task can do optimistic spinning only if the lock holder is
actually running. If the state of the lock holder isn't known, there
is a possibility that high priority of the RT task may block forward
progress of the lock holder if it happens to reside on the same CPU.
This will lead to deadlock. So we have to make sure that an RT task
will not spin on a reader-owned rwsem.

When the owner is temporarily set to NULL, there are two cases
where we may want to continue spinning:

 1) The lock owner is in the process of releasing the lock, sem->owner
    is cleared but the lock has not been released yet.

 2) The lock was free and owner cleared, but another task just comes
    in and acquire the lock before we try to get it. The new owner may
    be a spinnable writer.

So an RT task is now made to retry one more time to see if it can
acquire the lock or continue spinning on the new owning writer.

When testing on a 8-socket IvyBridge-EX system, the one additional retry
seems to improve locking performance of RT write locking threads under
heavy contentions. The table below shows the locking rates (in kops/s)
with various write locking threads before and after the patch.

    Locking threads     Pre-patch     Post-patch
    ---------------     ---------     -----------
            4             2,753          2,608
            8             2,529          2,520
           16             1,727          1,918
           32             1,263          1,956
           64               889          1,343
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: huang ying <huang.ying.caritas@gmail.com>
Link: https://lkml.kernel.org/r/20190520205918.22251-10-longman@redhat.comSigned-off-by: Ingo Molnar <mingo@kernel.org>