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

[Laurent Vivier <lvivier@redhat.com>]

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?

Thanks,
Laurent