I thought of a scenario where it seems appropriate to use a binary semaphore or
a mutex in a software interrupt context. If a device cannot interrupt when some
important variable changes, it can be polled occasionally to update e.g. LEDs
to indicate status. Such polling can be done most efficiently in timers that
are software interrupts. Timers are more efficient than works because they do
not have so much context switching overhead. If the access to the variables of
the device must be serialized, a binary semaphore or a mutex is a natural
choice. If user-space writing to the device is likely to change the status, it
can make sense not to poll the status of the device at the same time. The timer
could therefore sensibly call mutex_trylock. Therefore, it seems wrong to me to
deprecate binary semaphores and disallow the use of mutexes in software
interrupt contexts.
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