| Message ID | 1464976925-30525-1-git-send-email-will.deacon@arm.com |
|---|---|
| State | New |
| Headers | show |
diff --git a/arch/arm64/include/asm/spinlock.h b/arch/arm64/include/asm/spinlock.h index fc9682bfe002..16f6913f7dbc 100644 --- a/arch/arm64/include/asm/spinlock.h +++ b/arch/arm64/include/asm/spinlock.h @@ -148,6 +148,11 @@ static inline int arch_spin_value_unlocked(arch_spinlock_t lock) static inline int arch_spin_is_locked(arch_spinlock_t *lock) { + /* + * Ensure prior spin_lock operations to other locks have completed + * on this CPU before we test whether "lock" is locked. + */ + smp_mb(); return !arch_spin_value_unlocked(READ_ONCE(*lock)); }
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 51d7d5205d33 ("powerpc: Add smp_mb() to arch_spin_is_locked()") and we did something similar for spin_unlock_wait in d86b8da04dfa ("arm64: spinlock: serialise spin_unlock_wait against concurrent lockers"). This patch adds an smp_mb() to the start of our arch_spin_is_locked routine to ensure that the lock value is always loaded after any other locks have been taken by the current CPU.w Reported-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Will Deacon <will.deacon@arm.com> --- arch/arm64/include/asm/spinlock.h | 5 +++++ 1 file changed, 5 insertions(+) -- 2.1.4 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel