Concurrent migrate_disable() and set_cpus_allowed_ptr() has
interesting features. We rely on set_cpus_allowed_ptr() to not return
until the task runs inside the provided mask. This expectation is
exported to userspace.
This means that any set_cpus_allowed_ptr() caller must wait until
migrate_enable() allows migrations.
At the same time, we don't want migrate_enable() to schedule, due to
patterns like:
preempt_disable();
migrate_disable();
...
migrate_enable();
preempt_enable();
And:
raw_spin_lock(&B);
spin_unlock(&A);
this means that when migrate_enable() must restore the affinity
mask, it cannot wait for completion thereof. Luck will have it that
that is exactly the case where there is a pending
set_cpus_allowed_ptr(), so let that provide storage for the async stop
machine.
Much thanks to Valentin who used TLA+ most effective and found lots of
'interesting' cases.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Reviewed-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Link: https://lkml.kernel.org/r/20201023102346.921768277@infradead.org
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