locking/spinlocks/arm64: Remove smp_mb() from arch_spin_is_locked()

The following commit:

  38b850a730 ("arm64: spinlock: order spin_{is_locked,unlock_wait} against local locks")

... added an smp_mb() to arch_spin_is_locked(), in order
"to ensure that the lock value is always loaded after any other locks have
been taken by the current CPU", and reported one example (the "insane case"
in ipc/sem.c) relying on such guarantee.

It is however understood that spin_is_locked() is not required to provide
such an ordering guarantee (a guarantee that is currently not provided by
all the implementations/archs), and that callers relying on such ordering
should instead insert suitable memory barriers before acting on the result
of spin_is_locked().

Following a recent auditing [1] of the callers of {,raw_}spin_is_locked(),
revealing that none of them are relying on the ordering guarantee anymore,
this commit removes the leading smp_mb() from the primitive thus reverting
38b850a730.

[1] https://marc.info/?l=linux-kernel&m=151981440005264&w=2
    https://marc.info/?l=linux-kernel&m=152042843808540&w=2
    https://marc.info/?l=linux-kernel&m=152043346110262&w=2

Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: akiyks@gmail.com
Cc: boqun.feng@gmail.com
Cc: dhowells@redhat.com
Cc: j.alglave@ucl.ac.uk
Cc: linux-arch@vger.kernel.org
Cc: luc.maranget@inria.fr
Cc: npiggin@gmail.com
Cc: parri.andrea@gmail.com
Cc: stern@rowland.harvard.edu
Link: http://lkml.kernel.org/r/1526338889-7003-2-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/arm64/include/asm/spinlock.h

@@ -122,11 +122,6 @@ static inline int arch_spin_value_unlocked(arch_spinlock_t lock)
 
 static inline int arch_spin_is_locked(arch_spinlock_t *lock)
 {
-	/*
-	 * Ensure prior spin_lock operations to other locks have completed
-	 * on this CPU before we test whether "lock" is locked.
-	 */
-	smp_mb(); /* ^^^ */
 	return !arch_spin_value_unlocked(READ_ONCE(*lock));
 }