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Re: [PATCH] cpus: avoid stucking in pause_all_vcpus due to race
From: |
Paolo Bonzini |
Subject: |
Re: [PATCH] cpus: avoid stucking in pause_all_vcpus due to race |
Date: |
Mon, 16 Mar 2020 09:45:19 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 |
On 16/03/20 09:37, Longpeng(Mike) wrote:
> From: Longpeng <address@hidden>
>
> We found 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.
>
> As suggested by Paolo, resume_all_vcpus should notice this race, so we need
> to move the change of runstate before pause_all_vcpus in do_vm_stop() and
> ignore the resume request if runstate is not running.
>
> Cc: Paolo Bonzini <address@hidden>
> Cc: Dr . David Alan Gilbert <address@hidden>
> Cc: Richard Henderson <address@hidden>
> Signed-off-by: Longpeng <address@hidden>
Queued, thanks!
Paolo
> ---
> cpus.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/cpus.c b/cpus.c
> index b4f8b84..ef441bd 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -1026,9 +1026,9 @@ static int do_vm_stop(RunState state, bool send_stop)
> int ret = 0;
>
> if (runstate_is_running()) {
> + runstate_set(state);
> cpu_disable_ticks();
> pause_all_vcpus();
> - runstate_set(state);
> vm_state_notify(0, state);
> if (send_stop) {
> qapi_event_send_stop();
> @@ -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);
>