softmmu/cpus: Check if the cpu work list is empty atomically

Instead of taking the lock of the cpu work list in order to check if it's
empty, we can just read the head pointer atomically. This decreases
cpu_work_list_empty's share from 5% to 1.3% in a profile of icount-enabled
aarch64-softmmu.

Signed-off-by: Idan Horowitz <idan.horowitz@gmail.com>
Message-Id: <20220114004358.299534-1-idan.horowitz@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

softmmu/cpus.c

@@ -73,12 +73,7 @@ bool cpu_is_stopped(CPUState *cpu)
 
 bool cpu_work_list_empty(CPUState *cpu)
 {
-    bool ret;
-
-    qemu_mutex_lock(&cpu->work_mutex);
-    ret = QSIMPLEQ_EMPTY(&cpu->work_list);
-    qemu_mutex_unlock(&cpu->work_mutex);
-    return ret;
+    return QSIMPLEQ_EMPTY_ATOMIC(&cpu->work_list);
 }
 
 bool cpu_thread_is_idle(CPUState *cpu)