From f962cac4c24157aeceff59cbf9dac8b5e30c55da Mon Sep 17 00:00:00 2001 From: Longpeng Date: Mon, 16 Mar 2020 16:37:32 +0800 Subject: cpus: avoid pause_all_vcpus getting stuck due to race We found an issue when repeat reboot in guest during migration, it cause the migration thread never be waken up again.
| | 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. 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: Dr. David Alan Gilbert Cc: Richard Henderson Signed-off-by: Longpeng Suggested-by: Paolo Bonzini Message-Id: <20200316083732.2010-1-longpeng2@huawei.com> Signed-off-by: Paolo Bonzini --- cpus.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/cpus.c b/cpus.c index b4f8b84b61..ef441bdf62 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); -- cgit v1.2.3