x64: Fix rare SMP deadlock when acquiring stack

The SMP bringup code uses a linked list of stacks. The APs are all
brought up concurrently and so to acquire a stack, they compare and swap
on smp_stack_free. The current code works as long as the compare and
swap succeeds, if it fails, then smp_stack_free MUST be read again
otherwise the AP will be deadlocked. This patch fixes the current code
by enforcing a re-read on a cmpxchg failure.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Dan Schatzberg <schatzberg.dan@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

arch/x64/boot.S

@@ -175,8 +175,8 @@ trlock: .byte 0
 smpboot64:
 	mov smpboot_cr3, %rax
 	mov %rax, %cr3
-	mov smp_stack_free, %rax
 1:
+	mov smp_stack_free, %rax
 	mov 4096(%rax), %rbx
 	lock cmpxchg %rbx, smp_stack_free
 	jnz 1b