Re: [PATCH] tests/functional/test_x86_64_hotplug_cpu: Fix race condition during unplug

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Jan 07, 2025 at 12:52:45PM +0100, Thomas Huth wrote:
> When unplugging the CPU, the test tries to check for a successful
> unplug by changing to the /sys/devices/system/cpu/cpu1 directory
> to see whether that fails. However, the "cd" could be faster than
> the unplug operation in the kernel, so there is a race condition
> and the test sometimes fails here.
> Fix it by trying to change the directory in a loop until the the
> CPU has really been unplugged.
> 
> Reported-by: Stefan Hajnoczi <stefanha@gmail.com>
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  tests/functional/test_x86_64_hotplug_cpu.py | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/tests/functional/test_x86_64_hotplug_cpu.py b/tests/functional/test_x86_64_hotplug_cpu.py
> index b1d5156c72..7b9200ac2e 100755
> --- a/tests/functional/test_x86_64_hotplug_cpu.py
> +++ b/tests/functional/test_x86_64_hotplug_cpu.py
> @@ -59,11 +59,13 @@ def test_hotplug(self):
>                                            'cd /sys/devices/system/cpu/cpu1',
>                                            'cpu1#')
>  
> +        exec_command_and_wait_for_pattern(self, 'cd ..', prompt)

Is this actually needed ? Are we keeping the CPU from being unplugged by
being in that dir ? If so, why isn't it also needed in the while loop
below ?

>          self.vm.cmd('device_del', id='c1')
>  
>          exec_command_and_wait_for_pattern(self,
> -                                          'cd /sys/devices/system/cpu/cpu1',
> -                                          'No such file or directory')
> +                                    'while cd /sys/devices/system/cpu/cpu1 ;'
> +                                    ' do sleep 0.2 ; done',
> +                                    'No such file or directory')
>  
>  if __name__ == '__main__':
>      LinuxKernelTest.main()
> -- 
> 2.47.1
> 

With regards,
Daniel
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