Introduce a new lock for synchronization of the master->domains list, and use
that instead of the big master_sem lock, improving real-time performance of
fx. ecrt_domain_process() when EoE thread is being shut down (or other
use-cases where master_sem is held for extended periods).
The following locking order is used:
* master->master_sem
* master->domains_lock
* master->io_sem
* domain->datagram_pairs_lock
Meaning that it is allowed to take domains_lock while holding master_sem, and
it is allowed to hold domains_lock while locking io_sem and
datagram_pairs_lock.
[RV: For now, the lockdep_assert_held() annotations are commented out,
since they don't compile with CONFIG_PROVE_LOCKING and (the default)
--disable-rtmutex.]
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