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Diffstat (limited to 'fs/namei.c')
-rw-r--r--fs/namei.c90
1 files changed, 64 insertions, 26 deletions
diff --git a/fs/namei.c b/fs/namei.c
index 7720fbd5277b..2c30c84d4ea1 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -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();
}