locking/local_lock: Introduce localtry_lock_t

In !PREEMPT_RT local_lock_irqsave() disables interrupts to protect
critical section, but it doesn't prevent NMI, so the fully reentrant
code cannot use local_lock_irqsave() for exclusive access.

Introduce localtry_lock_t and localtry_lock_irqsave() that
disables interrupts and sets acquired=1, so localtry_lock_irqsave()
from NMI attempting to acquire the same lock will return false.

In PREEMPT_RT local_lock_irqsave() maps to preemptible spin_lock().
Map localtry_lock_irqsave() to preemptible spin_trylock().
When in hard IRQ or NMI return false right away, since
spin_trylock() is not safe due to explicit locking in the underneath
rt_spin_trylock() implementation. Removing this explicit locking and
attempting only "trylock" is undesired due to PI implications.

Note there is no need to use local_inc for acquired variable,
since it's a percpu variable with strict nesting scopes.

Acked-by: Davidlohr Bueso <[email protected]>
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Vlastimil Babka <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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