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 51d7d5205d ("powerpc: Add smp_mb() to
arch_spin_is_locked()") and, although we did something similar for
spin_unlock_wait in d86b8da04d ("arm64: spinlock: serialise
spin_unlock_wait against concurrent lockers") that doesn't actually take
care of ordering against local acquisition of a different lock.
This patch adds an smp_mb() to the start of our arch_spin_is_locked and
arch_spin_unlock_wait routines to ensure that the lock value is always
loaded after any other locks have been taken by the current CPU.
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
38b850a730