arm64: spinlock: serialise spin_unlock_wait against concurrent lockers

Boqun Feng reported a rather nasty ordering issue with spin_unlock_wait
on architectures implementing spin_lock with LL/SC sequences and acquire
semantics:

 | CPU 1                   CPU 2                     CPU 3
 | ==================      ====================      ==============
 |                                                   spin_unlock(&lock);
 |                         spin_lock(&lock):
 |                           r1 = *lock; // r1 == 0;
 |                         o = READ_ONCE(object); // reordered here
 | object = NULL;
 | smp_mb();
 | spin_unlock_wait(&lock);
 |                           *lock = 1;
 | smp_mb();
 | o->dead = true;
 |                         if (o) // true
 |                           BUG_ON(o->dead); // true!!

The crux of the problem is that spin_unlock_wait(&lock) can return on
CPU 1 whilst CPU 2 is in the process of taking the lock. This can be
resolved by upgrading spin_unlock_wait to a LOCK operation, forcing it
to serialise against a concurrent locker and giving it acquire semantics
in the process (although it is not at all clear whether this is needed -
different callers seem to assume different things about the barrier
semantics and architectures are similarly disjoint in their
implementations of the macro).

This patch implements spin_unlock_wait using an LL/SC sequence with
acquire semantics on arm64. For v8.1 systems with the LSE atomics, the
exclusive writeback is omitted, since the spin_lock operation is
indivisible and no intermediate state can be observed.
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/spinlock.h

@@ -26,9 +26,28 @@
  * The memory barriers are implicit with the load-acquire and store-release
  * instructions.
  */
+static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
+{
+	unsigned int tmp;
+	arch_spinlock_t lockval;
 
-#define arch_spin_unlock_wait(lock) \
-	do { while (arch_spin_is_locked(lock)) cpu_relax(); } while (0)
+	asm volatile(
+"	sevl\n"
+"1:	wfe\n"
+"2:	ldaxr	%w0, %2\n"
+"	eor	%w1, %w0, %w0, ror #16\n"
+"	cbnz	%w1, 1b\n"
+	ARM64_LSE_ATOMIC_INSN(
+	/* LL/SC */
+"	stxr	%w1, %w0, %2\n"
+"	cbnz	%w1, 2b\n", /* Serialise against any concurrent lockers */
+	/* LSE atomics */
+"	nop\n"
+"	nop\n")
+	: "=&r" (lockval), "=&r" (tmp), "+Q" (*lock)
+	:
+	: "memory");
+}
 
 #define arch_spin_lock_flags(lock, flags) arch_spin_lock(lock)