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spin_is_locked has grown two very different use-cases:
(1) [The sane case] API functions may require a certain lock to be held
by the caller and can therefore use spin_is_locked as part of an
assert statement in order to verify that the lock is indeed held.
For example, usage of assert_spin_locked.
(2) [The insane case] There are two locks, where a CPU takes one of the
locks and then checks whether or not the other one is held before
accessing some shared state. For example, the "optimized locking" in
ipc/sem.c.
In the latter case, the sequence looks like:
spin_lock(&sem->lock);
if (!spin_is_locked(&sma->sem_perm.lock))
/* Access shared state */
and requires that the spin_is_locked check is ordered after taking the
sem->lock. Unfortunately, since our spinlocks are implemented using a
LDAXR/STXR sequence, the read of &sma->sem_perm.lock can be speculated
before the STXR and consequently return a stale value.
Whilst this hasn't been seen to cause issues in practice, PowerPC fixed
the same issue in
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