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

[Thomas Huth]

On 07/01/2025 12.57, Daniel P. Berrangé wrote:
> 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 ?

I added it to make the console output less confusing (otherwise it's still 
shown in the prompt after the CPU has been unplugged)... but I can also 
remove this line again if you prefer it?

 Thomas