By looking at spinlocks and thinking better to the lock problems for
pps_events() I found this (possible) solution...

The pps_event() is now protected by a spinlock against
pps_register_source() and pps_unregister_source(), but since I cannot
disable IRQs I used the spin_trylock() into the pps_events() so even
if the process context holds the lock and an PPS event arrives (IRQ)
the system doesn't hang. Note that we cannot lose an event without
problems.

It could be more acceptable? I removed the "volatile" attribute! :)

Rodolfo

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