diff options
Diffstat (limited to 'kernel/locking/qspinlock.c')
| -rw-r--r-- | kernel/locking/qspinlock.c | 117 |
1 files changed, 0 insertions, 117 deletions
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c index fd24153e8a48..294294c71ba4 100644 --- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -268,123 +268,6 @@ static __always_inline u32 __pv_wait_head_or_lock(struct qspinlock *lock, #define queued_spin_lock_slowpath native_queued_spin_lock_slowpath #endif -/* - * Various notes on spin_is_locked() and spin_unlock_wait(), which are - * 'interesting' functions: - * - * PROBLEM: some architectures have an interesting issue with atomic ACQUIRE - * operations in that the ACQUIRE applies to the LOAD _not_ the STORE (ARM64, - * PPC). Also qspinlock has a similar issue per construction, the setting of - * the locked byte can be unordered acquiring the lock proper. - * - * This gets to be 'interesting' in the following cases, where the /should/s - * end up false because of this issue. - * - * - * CASE 1: - * - * So the spin_is_locked() correctness issue comes from something like: - * - * CPU0 CPU1 - * - * global_lock(); local_lock(i) - * spin_lock(&G) spin_lock(&L[i]) - * for (i) if (!spin_is_locked(&G)) { - * spin_unlock_wait(&L[i]); smp_acquire__after_ctrl_dep(); - * return; - * } - * // deal with fail - * - * Where it is important CPU1 sees G locked or CPU0 sees L[i] locked such - * that there is exclusion between the two critical sections. - * - * The load from spin_is_locked(&G) /should/ be constrained by the ACQUIRE from - * spin_lock(&L[i]), and similarly the load(s) from spin_unlock_wait(&L[i]) - * /should/ be constrained by the ACQUIRE from spin_lock(&G). - * - * Similarly, later stuff is constrained by the ACQUIRE from CTRL+RMB. - * - * - * CASE 2: - * - * For spin_unlock_wait() there is a second correctness issue, namely: - * - * CPU0 CPU1 - * - * flag = set; - * smp_mb(); spin_lock(&l) - * spin_unlock_wait(&l); if (!flag) - * // add to lockless list - * spin_unlock(&l); - * // iterate lockless list - * - * Which wants to ensure that CPU1 will stop adding bits to the list and CPU0 - * will observe the last entry on the list (if spin_unlock_wait() had ACQUIRE - * semantics etc..) - * - * Where flag /should/ be ordered against the locked store of l. - */ - -/* - * queued_spin_lock_slowpath() can (load-)ACQUIRE the lock before - * issuing an _unordered_ store to set _Q_LOCKED_VAL. - * - * This means that the store can be delayed, but no later than the - * store-release from the unlock. This means that simply observing - * _Q_LOCKED_VAL is not sufficient to determine if the lock is acquired. - * - * There are two paths that can issue the unordered store: - * - * (1) clear_pending_set_locked(): *,1,0 -> *,0,1 - * - * (2) set_locked(): t,0,0 -> t,0,1 ; t != 0 - * atomic_cmpxchg_relaxed(): t,0,0 -> 0,0,1 - * - * However, in both cases we have other !0 state we've set before to queue - * ourseves: - * - * For (1) we have the atomic_cmpxchg_acquire() that set _Q_PENDING_VAL, our - * load is constrained by that ACQUIRE to not pass before that, and thus must - * observe the store. - * - * For (2) we have a more intersting scenario. We enqueue ourselves using - * xchg_tail(), which ends up being a RELEASE. This in itself is not - * sufficient, however that is followed by an smp_cond_acquire() on the same - * word, giving a RELEASE->ACQUIRE ordering. This again constrains our load and - * guarantees we must observe that store. - * - * Therefore both cases have other !0 state that is observable before the - * unordered locked byte store comes through. This means we can use that to - * wait for the lock store, and then wait for an unlock. - */ -#ifndef queued_spin_unlock_wait -void queued_spin_unlock_wait(struct qspinlock *lock) -{ - u32 val; - - for (;;) { - val = atomic_read(&lock->val); - - if (!val) /* not locked, we're done */ - goto done; - - if (val & _Q_LOCKED_MASK) /* locked, go wait for unlock */ - break; - - /* not locked, but pending, wait until we observe the lock */ - cpu_relax(); - } - - /* any unlock is good */ - while (atomic_read(&lock->val) & _Q_LOCKED_MASK) - cpu_relax(); - -done: - smp_acquire__after_ctrl_dep(); -} -EXPORT_SYMBOL(queued_spin_unlock_wait); -#endif - #endif /* _GEN_PV_LOCK_SLOWPATH */ /** |