diff options
| -rw-r--r-- | fs/dcache.c | 2 | ||||
| -rw-r--r-- | fs/namei.c | 90 | ||||
| -rw-r--r-- | include/linux/dcache.h | 22 |
3 files changed, 65 insertions, 49 deletions
diff --git a/fs/dcache.c b/fs/dcache.c index 2d24422..96655f4 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -1786,7 +1786,7 @@ static noinline enum slow_d_compare slow_dentry_cmp( * without taking d_lock and checking d_seq sequence count against @seq * returned here. * - * A refcount may be taken on the found dentry with the __d_rcu_to_refcount + * A refcount may be taken on the found dentry with the d_rcu_to_refcount * function. * * Alternatively, __d_lookup_rcu may be called again to look up the child of @@ -494,6 +494,50 @@ static inline void unlock_rcu_walk(void) br_read_unlock(&vfsmount_lock); } +/* + * When we move over from the RCU domain to properly refcounted + * long-lived dentries, we need to check the sequence numbers + * we got before lookup very carefully. + * + * We cannot blindly increment a dentry refcount - even if it + * is not locked - if it is zero, because it may have gone + * through the final d_kill() logic already. + * + * So for a zero refcount, we need to get the spinlock (which is + * safe even for a dead dentry because the de-allocation is + * RCU-delayed), and check the sequence count under the lock. + * + * Once we have checked the sequence count, we know it is live, + * and since we hold the spinlock it cannot die from under us. + * + * In contrast, if the reference count wasn't zero, we can just + * increment the lockref without having to take the spinlock. + * Even if the sequence number ends up being stale, we haven't + * gone through the final dput() and killed the dentry yet. + */ +static inline int d_rcu_to_refcount(struct dentry *dentry, seqcount_t *validate, unsigned seq) +{ + int gotref; + + gotref = lockref_get_or_lock(&dentry->d_lockref); + + /* Does the sequence number still match? */ + if (read_seqcount_retry(validate, seq)) { + if (gotref) + dput(dentry); + else + spin_unlock(&dentry->d_lock); + return -ECHILD; + } + + /* Get the ref now, if we couldn't get it originally */ + if (!gotref) { + dentry->d_lockref.count++; + spin_unlock(&dentry->d_lock); + } + return 0; +} + /** * unlazy_walk - try to switch to ref-walk mode. * @nd: nameidata pathwalk data @@ -518,29 +562,28 @@ static int unlazy_walk(struct nameidata *nd, struct dentry *dentry) nd->root.dentry != fs->root.dentry) goto err_root; } - spin_lock(&parent->d_lock); + + /* + * For a negative lookup, the lookup sequence point is the parents + * sequence point, and it only needs to revalidate the parent dentry. + * + * For a positive lookup, we need to move both the parent and the + * dentry from the RCU domain to be properly refcounted. And the + * sequence number in the dentry validates *both* dentry counters, + * since we checked the sequence number of the parent after we got + * the child sequence number. So we know the parent must still + * be valid if the child sequence number is still valid. + */ if (!dentry) { - if (!__d_rcu_to_refcount(parent, nd->seq)) - goto err_parent; + if (d_rcu_to_refcount(parent, &parent->d_seq, nd->seq) < 0) + goto err_root; BUG_ON(nd->inode != parent->d_inode); } else { - if (dentry->d_parent != parent) + if (d_rcu_to_refcount(dentry, &dentry->d_seq, nd->seq) < 0) + goto err_root; + if (d_rcu_to_refcount(parent, &dentry->d_seq, nd->seq) < 0) goto err_parent; - spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED); - if (!__d_rcu_to_refcount(dentry, nd->seq)) - goto err_child; - /* - * If the sequence check on the child dentry passed, then - * the child has not been removed from its parent. This - * means the parent dentry must be valid and able to take - * a reference at this point. - */ - BUG_ON(!IS_ROOT(dentry) && dentry->d_parent != parent); - BUG_ON(!parent->d_lockref.count); - parent->d_lockref.count++; - spin_unlock(&dentry->d_lock); } - spin_unlock(&parent->d_lock); if (want_root) { path_get(&nd->root); spin_unlock(&fs->lock); @@ -551,10 +594,8 @@ static int unlazy_walk(struct nameidata *nd, struct dentry *dentry) nd->flags &= ~LOOKUP_RCU; return 0; -err_child: - spin_unlock(&dentry->d_lock); err_parent: - spin_unlock(&parent->d_lock); + dput(dentry); err_root: if (want_root) spin_unlock(&fs->lock); @@ -585,14 +626,11 @@ static int complete_walk(struct nameidata *nd) nd->flags &= ~LOOKUP_RCU; if (!(nd->flags & LOOKUP_ROOT)) nd->root.mnt = NULL; - spin_lock(&dentry->d_lock); - if (unlikely(!__d_rcu_to_refcount(dentry, nd->seq))) { - spin_unlock(&dentry->d_lock); + + if (d_rcu_to_refcount(dentry, &dentry->d_seq, nd->seq) < 0) { unlock_rcu_walk(); return -ECHILD; } - BUG_ON(nd->inode != dentry->d_inode); - spin_unlock(&dentry->d_lock); mntget(nd->path.mnt); unlock_rcu_walk(); } diff --git a/include/linux/dcache.h b/include/linux/dcache.h index efdc944..9169b91 100644 --- a/include/linux/dcache.h +++ b/include/linux/dcache.h @@ -304,28 +304,6 @@ extern struct dentry *__d_lookup(const struct dentry *, const struct qstr *); extern struct dentry *__d_lookup_rcu(const struct dentry *parent, const struct qstr *name, unsigned *seq); -/** - * __d_rcu_to_refcount - take a refcount on dentry if sequence check is ok - * @dentry: dentry to take a ref on - * @seq: seqcount to verify against - * Returns: 0 on failure, else 1. - * - * __d_rcu_to_refcount operates on a dentry,seq pair that was returned - * by __d_lookup_rcu, to get a reference on an rcu-walk dentry. - */ -static inline int __d_rcu_to_refcount(struct dentry *dentry, unsigned seq) -{ - int ret = 0; - - assert_spin_locked(&dentry->d_lock); - if (!read_seqcount_retry(&dentry->d_seq, seq)) { - ret = 1; - dentry->d_lockref.count++; - } - - return ret; -} - static inline unsigned d_count(const struct dentry *dentry) { return dentry->d_lockref.count; |
