On 10/03/20 10:14, Longpeng(Mike) wrote:
> From: Longpeng <longpe...@huawei.com>
>
> We find an issue when repeat reboot in guest during migration, it cause the
> migration thread never be waken up again.
>
> <main loop> |<migration_thread>
> |
> LOCK BQL |
> ... |
> main_loop_should_exit |
> pause_all_vcpus |
> 1. set all cpus ->stop=true |
> and then kick |
> 2. return if all cpus is paused |
> (by '->stopped == true'), else|
> 3. qemu_cond_wait [BQL UNLOCK] |
> |LOCK BQL
> |...
> |do_vm_stop
> | pause_all_vcpus
> | (A)set all cpus ->stop=true
> | and then kick
> | (B)return if all cpus is paused
> | (by '->stopped == true'), else
> | (C)qemu_cond_wait [BQL UNLOCK]
> 4. be waken up and LOCK BQL | (D)be waken up BUT wait for BQL
> 5. goto 2. |
> (BQL is still LOCKed) |
> resume_all_vcpus |
> 1. set all cpus ->stop=false |
> and ->stopped=false |
> ... |
> BQL UNLOCK | (E)LOCK BQL
> | (F)goto B. [but stopped is false now!]
> |Finally, sleep at step 3 forever.
>
>
> Note: This patch is just for discuss this issue, I'm looking forward to
> your suggestions, thanks!
Thanks Mike,
the above sketch is really helpful.
I think the problem is not that pause_all_vcpus() is not pausing hard
enough; the problem is rather than resume_all_vcpus(), when used outside
vm_start(), should know about the race and do nothing if it happens.
Fortunately resume_all_vcpus does not release the BQL so it should be
enough to test once; translated to code, this would be the patch to fix it:
diff --git a/cpus.c b/cpus.c
index b4f8b84b61..1eb7533a91 100644
--- a/cpus.c
+++ b/cpus.c
@@ -1899,6 +1899,10 @@ void resume_all_vcpus(void)
{
CPUState *cpu;
+ if (!runstate_is_running()) {
+ return;
+ }
+
qemu_clock_enable(QEMU_CLOCK_VIRTUAL, true);
CPU_FOREACH(cpu) {
cpu_resume(cpu);
Thanks,
Paolo
