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Subjectcalling smp_call_function_many() with non-stable CPU mask
There are a couple of examples of smp_call_function_many() getting
called with mm_cpumask() as the first argument. Since that mask
generally can change while smp_call_function_many() is executing,
it seems there might be a problem with the case where that mask
becomes empty after the initial checks, but before the mask is made
permanent (by copying into data->cpumask).

Shouldn't there be a check of data->refs being zero right after
setting it (to avoid having csd_lock_wait() wait for a remote CPU
to clear the lock flag, and to avoid adding the entry to

If that isn't considered necessary, is it then incorrect to pass
in-flight CPU masks to smp_call_function_many() (and should
this requirement then be documented somewhere, and the
existing calls all be inspected for correctness)?

Thanks, Jan

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