diff options
| -rw-r--r-- | kernel/futex.c | 117 |
1 files changed, 92 insertions, 25 deletions
diff --git a/kernel/futex.c b/kernel/futex.c index fcc6850..30971b5 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -75,17 +75,20 @@ * The waiter reads the futex value in user space and calls * futex_wait(). This function computes the hash bucket and acquires * the hash bucket lock. After that it reads the futex user space value - * again and verifies that the data has not changed. If it has not - * changed it enqueues itself into the hash bucket, releases the hash - * bucket lock and schedules. + * again and verifies that the data has not changed. If it has not changed + * it enqueues itself into the hash bucket, releases the hash bucket lock + * and schedules. * * The waker side modifies the user space value of the futex and calls - * futex_wake(). This functions computes the hash bucket and acquires - * the hash bucket lock. Then it looks for waiters on that futex in the - * hash bucket and wakes them. + * futex_wake(). This function computes the hash bucket and acquires the + * hash bucket lock. Then it looks for waiters on that futex in the hash + * bucket and wakes them. * - * Note that the spin_lock serializes waiters and wakers, so that the - * following scenario is avoided: + * In futex wake up scenarios where no tasks are blocked on a futex, taking + * the hb spinlock can be avoided and simply return. In order for this + * optimization to work, ordering guarantees must exist so that the waiter + * being added to the list is acknowledged when the list is concurrently being + * checked by the waker, avoiding scenarios like the following: * * CPU 0 CPU 1 * val = *futex; @@ -106,24 +109,52 @@ * This would cause the waiter on CPU 0 to wait forever because it * missed the transition of the user space value from val to newval * and the waker did not find the waiter in the hash bucket queue. - * The spinlock serializes that: * - * CPU 0 CPU 1 + * The correct serialization ensures that a waiter either observes + * the changed user space value before blocking or is woken by a + * concurrent waker: + * + * CPU 0 CPU 1 * val = *futex; * sys_futex(WAIT, futex, val); * futex_wait(futex, val); - * lock(hash_bucket(futex)); - * uval = *futex; - * *futex = newval; - * sys_futex(WAKE, futex); - * futex_wake(futex); - * lock(hash_bucket(futex)); + * + * waiters++; + * mb(); (A) <-- paired with -. + * | + * lock(hash_bucket(futex)); | + * | + * uval = *futex; | + * | *futex = newval; + * | sys_futex(WAKE, futex); + * | futex_wake(futex); + * | + * `-------> mb(); (B) * if (uval == val) - * queue(); + * queue(); * unlock(hash_bucket(futex)); - * schedule(); if (!queue_empty()) - * wake_waiters(futex); - * unlock(hash_bucket(futex)); + * schedule(); if (waiters) + * lock(hash_bucket(futex)); + * wake_waiters(futex); + * unlock(hash_bucket(futex)); + * + * Where (A) orders the waiters increment and the futex value read -- this + * is guaranteed by the head counter in the hb spinlock; and where (B) + * orders the write to futex and the waiters read -- this is done by the + * barriers in get_futex_key_refs(), through either ihold or atomic_inc, + * depending on the futex type. + * + * This yields the following case (where X:=waiters, Y:=futex): + * + * X = Y = 0 + * + * w[X]=1 w[Y]=1 + * MB MB + * r[Y]=y r[X]=x + * + * Which guarantees that x==0 && y==0 is impossible; which translates back into + * the guarantee that we cannot both miss the futex variable change and the + * enqueue. */ int __read_mostly futex_cmpxchg_enabled; @@ -211,6 +242,36 @@ static unsigned long __read_mostly futex_hashsize; static struct futex_hash_bucket *futex_queues; +static inline void futex_get_mm(union futex_key *key) +{ + atomic_inc(&key->private.mm->mm_count); + /* + * Ensure futex_get_mm() implies a full barrier such that + * get_futex_key() implies a full barrier. This is relied upon + * as full barrier (B), see the ordering comment above. + */ + smp_mb__after_atomic_inc(); +} + +static inline bool hb_waiters_pending(struct futex_hash_bucket *hb) +{ +#ifdef CONFIG_SMP + /* + * Tasks trying to enter the critical region are most likely + * potential waiters that will be added to the plist. Ensure + * that wakers won't miss to-be-slept tasks in the window between + * the wait call and the actual plist_add. + */ + if (spin_is_locked(&hb->lock)) + return true; + smp_rmb(); /* Make sure we check the lock state first */ + + return !plist_head_empty(&hb->chain); +#else + return true; +#endif +} + /* * We hash on the keys returned from get_futex_key (see below). */ @@ -245,10 +306,10 @@ static void get_futex_key_refs(union futex_key *key) switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) { case FUT_OFF_INODE: - ihold(key->shared.inode); + ihold(key->shared.inode); /* implies MB (B) */ break; case FUT_OFF_MMSHARED: - atomic_inc(&key->private.mm->mm_count); + futex_get_mm(key); /* implies MB (B) */ break; } } @@ -322,7 +383,7 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw) if (!fshared) { key->private.mm = mm; key->private.address = address; - get_futex_key_refs(key); + get_futex_key_refs(key); /* implies MB (B) */ return 0; } @@ -429,7 +490,7 @@ again: key->shared.pgoff = basepage_index(page); } - get_futex_key_refs(key); + get_futex_key_refs(key); /* implies MB (B) */ out: unlock_page(page_head); @@ -1052,6 +1113,11 @@ futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset) goto out; hb = hash_futex(&key); + + /* Make sure we really have tasks to wakeup */ + if (!hb_waiters_pending(hb)) + goto out_put_key; + spin_lock(&hb->lock); plist_for_each_entry_safe(this, next, &hb->chain, list) { @@ -1072,6 +1138,7 @@ futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset) } spin_unlock(&hb->lock); +out_put_key: put_futex_key(&key); out: return ret; @@ -1535,7 +1602,7 @@ static inline struct futex_hash_bucket *queue_lock(struct futex_q *q) hb = hash_futex(&q->key); q->lock_ptr = &hb->lock; - spin_lock(&hb->lock); + spin_lock(&hb->lock); /* implies MB (A) */ return hb; } |
