tcg: synchronize cpu->exit_request and cpu->tcg_exit_req accesses

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

qom/cpu.c

@@ -114,6 +114,8 @@ void cpu_reset_interrupt(CPUState *cpu, int mask)
 void cpu_exit(CPUState *cpu)
 {
     cpu->exit_request = 1;
+    /* Ensure cpu_exec will see the exit request after TCG has exited.  */
+    smp_wmb();
     cpu->tcg_exit_req = 1;
 }