Re: [Qemu-devel] [PATCH] cpus: ignore ESRCH in qemu_cpu_kick_thread()

[Laurent Vivier <lvivier@redhat.com>]

On 08/01/2019 19:47, Laurent Vivier wrote:
> On 08/01/2019 00:02, Paolo Bonzini wrote:
>> On 02/01/19 15:16, Laurent Vivier wrote:
>>> We can have a race condition between qemu_cpu_kick_thread() and
>>> qemu_kvm_cpu_thread_fn() when we hotunplug a CPU. In this case,
>>> qemu_cpu_kick_thread() can try to kick a thread that is exiting.
>>> pthread_kill() returns an error and qemu is stopped by an exit(1).
>>>
>>>     qemu:qemu_cpu_kick_thread: No such process
>>>
>>> We can ignore safely this error.
>>>
>>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
>>> ---
>>>   cpus.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/cpus.c b/cpus.c
>>> index 0ddeeefc14..4717490bd0 100644
>>> --- a/cpus.c
>>> +++ b/cpus.c
>>> @@ -1778,7 +1778,7 @@ static void qemu_cpu_kick_thread(CPUState *cpu)
>>>       }
>>>       cpu->thread_kicked = true;
>>>       err = pthread_kill(cpu->thread->thread, SIG_IPI);
>>> -    if (err) {
>>> +    if (err && err != ESRCH) {
>>>           fprintf(stderr, "qemu:%s: %s", __func__, strerror(err));
>>>           exit(1);
>>>       }
>>>
>>
>> You could in principle be sending the signal to another thread, so the
>> fix is a bit hackish.  However, I don't have a better idea that is not
>> racy. :(
>>
>> The problem is that qemu_cpu_kick does not use any spinlock or mutex to
>> synchronize against cpu_remove_sync's qemu_thread_join.  I think once
>> the you reach qemu_cpu_kick in cpu_remove_sync (so if cpu->unplug) you
>> do not need to reset cpu->thread_kicked anymore, but I don't think
>> that's enough to fix it.
> 
> Will you take the patch through one of your pull requests or should I
> add it to the trivial-patches branch?

Paolo?

Thanks,
Laurent