locking/qspinlock: Ensure node is initialised before updating prev->next

When a locker ends up queuing on the qspinlock locking slowpath, we
initialise the relevant mcs node and publish it indirectly by updating
the tail portion of the lock word using xchg_tail. If we find that there
was a pre-existing locker in the queue, we subsequently update their
->next field to point at our node so that we are notified when it's our
turn to take the lock.

This can be roughly illustrated as follows:

  /* Initialise the fields in node and encode a pointer to node in tail */
  tail = initialise_node(node);

  /*
   * Exchange tail into the lockword using an atomic read-modify-write
   * operation with release semantics
   */
  old = xchg_tail(lock, tail);

  /* If there was a pre-existing waiter ... */
  if (old & _Q_TAIL_MASK) {
	prev = decode_tail(old);
	smp_read_barrier_depends();

	/* ... then update their ->next field to point to node.
	WRITE_ONCE(prev->next, node);
  }

The conditional update of prev->next therefore relies on the address
dependency from the result of xchg_tail ensuring order against the
prior initialisation of node. However, since the release semantics of
the xchg_tail operation apply only to the write portion of the RmW,
then this ordering is not guaranteed and it is possible for the CPU
to return old before the writes to node have been published, consequently
allowing us to point prev->next to an uninitialised node.

This patch fixes the problem by making the update of prev->next a RELEASE
operation, which also removes the reliance on dependency ordering.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1518528177-19169-2-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>