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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. <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. 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 <dgilbert@redhat.com> Cc: Richard Henderson <rth@twiddle.net> Signed-off-by: Longpeng <longpeng2@huawei.com> Suggested-by: Paolo Bonzini <pbonzini@redhat.com> Message-Id: <20200316083732.2010-1-longpeng2@huawei.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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parent
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6
cpus.c
6
cpus.c
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@ -1026,9 +1026,9 @@ static int do_vm_stop(RunState state, bool send_stop)
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int ret = 0;
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if (runstate_is_running()) {
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runstate_set(state);
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cpu_disable_ticks();
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pause_all_vcpus();
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runstate_set(state);
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vm_state_notify(0, state);
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if (send_stop) {
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qapi_event_send_stop();
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@ -1899,6 +1899,10 @@ void resume_all_vcpus(void)
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{
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CPUState *cpu;
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if (!runstate_is_running()) {
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return;
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}
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qemu_clock_enable(QEMU_CLOCK_VIRTUAL, true);
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CPU_FOREACH(cpu) {
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cpu_resume(cpu);
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