Commit 38b850a7 authored by Will Deacon's avatar Will Deacon

arm64: spinlock: order spin_{is_locked,unlock_wait} against local locks

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 51d7d520 ("powerpc: Add smp_mb() to
arch_spin_is_locked()") and, although we did something similar for
spin_unlock_wait in d86b8da0 ("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: default avatarPeter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
parent f7a6c149
......@@ -31,6 +31,12 @@ static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
unsigned int tmp;
arch_spinlock_t lockval;
/*
* Ensure prior spin_lock operations to other locks have completed
* on this CPU before we test whether "lock" is locked.
*/
smp_mb();
asm volatile(
" sevl\n"
"1: wfe\n"
......@@ -148,6 +154,7 @@ static inline int arch_spin_value_unlocked(arch_spinlock_t lock)
static inline int arch_spin_is_locked(arch_spinlock_t *lock)
{
smp_mb(); /* See arch_spin_unlock_wait */
return !arch_spin_value_unlocked(READ_ONCE(*lock));
}
......
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