fs: Use rename lock and RCU for multi-step operations

The remaining usages for dcache_lock is to allow atomic, multi-step read-side
operations over the directory tree by excluding modifications to the tree.
Also, to walk in the leaf->root direction in the tree where we don't have
a natural d_lock ordering.

This could be accomplished by taking every d_lock, but this would mean a
huge number of locks and actually gets very tricky.

Solve this instead by using the rename seqlock for multi-step read-side
operations, retry in case of a rename so we don't walk up the wrong parent.
Concurrent dentry insertions are not serialised against.  Concurrent deletes
are tricky when walking up the directory: our parent might have been deleted
when dropping locks so also need to check and retry for that.

We can also use the rename lock in cases where livelock is a worry (and it
is introduced in subsequent patch).

Signed-off-by: Nick Piggin <npiggin@kernel.dk>

1 files changed, 111 insertions, 23 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index a09f0771fd27..a9bc4ecc21e1 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -80,6 +80,7 @@ static __cacheline_aligned_in_smp DEFINE_SPINLOCK(dcache_lru_lock);
 __cacheline_aligned_in_smp DEFINE_SPINLOCK(dcache_lock);
 __cacheline_aligned_in_smp DEFINE_SEQLOCK(rename_lock);
 
+EXPORT_SYMBOL(rename_lock);
 EXPORT_SYMBOL(dcache_inode_lock);
 EXPORT_SYMBOL(dcache_lock);
 
@@ -243,6 +244,7 @@ static struct dentry *d_kill(struct dentry *dentry, struct dentry *parent)
         __releases(dcache_inode_lock)
         __releases(dcache_lock)
 {
+        dentry->d_parent = NULL;
         list_del(&dentry->d_u.d_child);
         if (parent)
                 spin_unlock(&parent->d_lock);
@@ -1017,11 +1019,15 @@ void shrink_dcache_for_umount(struct super_block *sb)
  * Return true if the parent or its subdirectories contain
  * a mount point
  */
- 
 int have_submounts(struct dentry *parent)
 {
-        struct dentry *this_parent = parent;
+        struct dentry *this_parent;
         struct list_head *next;
+        unsigned seq;
+
+rename_retry:
+        this_parent = parent;
+        seq = read_seqbegin(&rename_lock);
 
         spin_lock(&dcache_lock);
         if (d_mountpoint(parent))
@@ -1055,17 +1061,37 @@ resume:
          * All done at this level ... ascend and resume the search.
          */
         if (this_parent != parent) {
-                next = this_parent->d_u.d_child.next;
+                struct dentry *tmp;
+                struct dentry *child;
+
+                tmp = this_parent->d_parent;
+                rcu_read_lock();
                 spin_unlock(&this_parent->d_lock);
-                this_parent = this_parent->d_parent;
+                child = this_parent;
+                this_parent = tmp;
                 spin_lock(&this_parent->d_lock);
+                /* might go back up the wrong parent if we have had a rename
+                 * or deletion */
+                if (this_parent != child->d_parent ||
+                                read_seqretry(&rename_lock, seq)) {
+                        spin_unlock(&this_parent->d_lock);
+                        spin_unlock(&dcache_lock);
+                        rcu_read_unlock();
+                        goto rename_retry;
+                }
+                rcu_read_unlock();
+                next = child->d_u.d_child.next;
                 goto resume;
         }
         spin_unlock(&this_parent->d_lock);
         spin_unlock(&dcache_lock);
+        if (read_seqretry(&rename_lock, seq))
+                goto rename_retry;
         return 0; /* No mount points found in tree */
 positive:
         spin_unlock(&dcache_lock);
+        if (read_seqretry(&rename_lock, seq))
+                goto rename_retry;
         return 1;
 }
 EXPORT_SYMBOL(have_submounts);
@@ -1086,10 +1112,15 @@ EXPORT_SYMBOL(have_submounts);
  */
 static int select_parent(struct dentry * parent)
 {
-        struct dentry *this_parent = parent;
+        struct dentry *this_parent;
         struct list_head *next;
+        unsigned seq;
         int found = 0;
 
+rename_retry:
+        this_parent = parent;
+        seq = read_seqbegin(&rename_lock);
+
         spin_lock(&dcache_lock);
         spin_lock(&this_parent->d_lock);
 repeat:
@@ -1099,7 +1130,6 @@ resume:
                 struct list_head *tmp = next;
                 struct dentry *dentry = list_entry(tmp, struct dentry, d_u.d_child);
                 next = tmp->next;
-                BUG_ON(this_parent == dentry);
 
                 spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
 
@@ -1142,17 +1172,32 @@ resume:
          */
         if (this_parent != parent) {
                 struct dentry *tmp;
-                next = this_parent->d_u.d_child.next;
+                struct dentry *child;
+
                 tmp = this_parent->d_parent;
+                rcu_read_lock();
                 spin_unlock(&this_parent->d_lock);
-                BUG_ON(tmp == this_parent);
+                child = this_parent;
                 this_parent = tmp;
                 spin_lock(&this_parent->d_lock);
+                /* might go back up the wrong parent if we have had a rename
+                 * or deletion */
+                if (this_parent != child->d_parent ||
+                                read_seqretry(&rename_lock, seq)) {
+                        spin_unlock(&this_parent->d_lock);
+                        spin_unlock(&dcache_lock);
+                        rcu_read_unlock();
+                        goto rename_retry;
+                }
+                rcu_read_unlock();
+                next = child->d_u.d_child.next;
                 goto resume;
         }
 out:
         spin_unlock(&this_parent->d_lock);
         spin_unlock(&dcache_lock);
+        if (read_seqretry(&rename_lock, seq))
+                goto rename_retry;
         return found;
 }
 
@@ -1654,7 +1699,7 @@ EXPORT_SYMBOL(d_add_ci);
 struct dentry * d_lookup(struct dentry * parent, struct qstr * name)
 {
         struct dentry * dentry = NULL;
-        unsigned long seq;
+        unsigned seq;
 
         do {
                 seq = read_seqbegin(&rename_lock);
@@ -2290,7 +2335,7 @@ static int prepend_name(char **buffer, int *buflen, struct qstr *name)
  * @buffer: pointer to the end of the buffer
  * @buflen: pointer to buffer length
  *
- * Caller holds the dcache_lock.
+ * Caller holds the rename_lock.
  *
  * If path is not reachable from the supplied root, then the value of
  * root is changed (without modifying refcounts).
@@ -2377,7 +2422,9 @@ char *__d_path(const struct path *path, struct path *root,
 
         prepend(&res, &buflen, "\0", 1);
         spin_lock(&dcache_lock);
+        write_seqlock(&rename_lock);
         error = prepend_path(path, root, &res, &buflen);
+        write_sequnlock(&rename_lock);
         spin_unlock(&dcache_lock);
 
         if (error)
@@ -2441,10 +2488,12 @@ char *d_path(const struct path *path, char *buf, int buflen)
 
         get_fs_root(current->fs, &root);
         spin_lock(&dcache_lock);
+        write_seqlock(&rename_lock);
         tmp = root;
         error = path_with_deleted(path, &tmp, &res, &buflen);
         if (error)
                 res = ERR_PTR(error);
+        write_sequnlock(&rename_lock);
         spin_unlock(&dcache_lock);
         path_put(&root);
         return res;
@@ -2472,10 +2521,12 @@ char *d_path_with_unreachable(const struct path *path, char *buf, int buflen)
 
         get_fs_root(current->fs, &root);
         spin_lock(&dcache_lock);
+        write_seqlock(&rename_lock);
         tmp = root;
         error = path_with_deleted(path, &tmp, &res, &buflen);
         if (!error && !path_equal(&tmp, &root))
                 error = prepend_unreachable(&res, &buflen);
+        write_sequnlock(&rename_lock);
         spin_unlock(&dcache_lock);
         path_put(&root);
         if (error)
@@ -2544,7 +2595,9 @@ char *dentry_path_raw(struct dentry *dentry, char *buf, int buflen)
         char *retval;
 
         spin_lock(&dcache_lock);
+        write_seqlock(&rename_lock);
         retval = __dentry_path(dentry, buf, buflen);
+        write_sequnlock(&rename_lock);
         spin_unlock(&dcache_lock);
 
         return retval;
@@ -2557,6 +2610,7 @@ char *dentry_path(struct dentry *dentry, char *buf, int buflen)
         char *retval;
 
         spin_lock(&dcache_lock);
+        write_seqlock(&rename_lock);
         if (d_unlinked(dentry)) {
                 p = buf + buflen;
                 if (prepend(&p, &buflen, "//deleted", 10) != 0)
@@ -2564,6 +2618,7 @@ char *dentry_path(struct dentry *dentry, char *buf, int buflen)
                 buflen++;
         }
         retval = __dentry_path(dentry, buf, buflen);
+        write_sequnlock(&rename_lock);
         spin_unlock(&dcache_lock);
         if (!IS_ERR(retval) && p)
                 *p = '/';       /* restore '/' overriden with '\0' */
@@ -2604,6 +2659,7 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
 
         error = -ENOENT;
         spin_lock(&dcache_lock);
+        write_seqlock(&rename_lock);
         if (!d_unlinked(pwd.dentry)) {
                 unsigned long len;
                 struct path tmp = root;
@@ -2612,6 +2668,7 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
 
                 prepend(&cwd, &buflen, "\0", 1);
                 error = prepend_path(&pwd, &tmp, &cwd, &buflen);
+                write_sequnlock(&rename_lock);
                 spin_unlock(&dcache_lock);
 
                 if (error)
@@ -2631,8 +2688,10 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
                         if (copy_to_user(buf, cwd, len))
                                 error = -EFAULT;
                 }
-        } else
+        } else {
+                write_sequnlock(&rename_lock);
                 spin_unlock(&dcache_lock);
+        }
 
 out:
         path_put(&pwd);
@@ -2660,25 +2719,25 @@ out:
 int is_subdir(struct dentry *new_dentry, struct dentry *old_dentry)
 {
         int result;
-        unsigned long seq;
+        unsigned seq;
 
         if (new_dentry == old_dentry)
                 return 1;
 
-        /*
-         * Need rcu_readlock to protect against the d_parent trashing
-         * due to d_move
-         */
-        rcu_read_lock();
         do {
                 /* for restarting inner loop in case of seq retry */
                 seq = read_seqbegin(&rename_lock);
+                /*
+                 * Need rcu_readlock to protect against the d_parent trashing
+                 * due to d_move
+                 */
+                rcu_read_lock();
                 if (d_ancestor(old_dentry, new_dentry))
                         result = 1;
                 else
                         result = 0;
+                rcu_read_unlock();
         } while (read_seqretry(&rename_lock, seq));
-        rcu_read_unlock();
 
         return result;
 }
@@ -2710,9 +2769,13 @@ EXPORT_SYMBOL(path_is_under);
 
 void d_genocide(struct dentry *root)
 {
-        struct dentry *this_parent = root;
+        struct dentry *this_parent;
         struct list_head *next;
+        unsigned seq;
 
+rename_retry:
+        this_parent = root;
+        seq = read_seqbegin(&rename_lock);
         spin_lock(&dcache_lock);
         spin_lock(&this_parent->d_lock);
 repeat:
@@ -2722,6 +2785,7 @@ resume:
                 struct list_head *tmp = next;
                 struct dentry *dentry = list_entry(tmp, struct dentry, d_u.d_child);
                 next = tmp->next;
+
                 spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
                 if (d_unhashed(dentry) || !dentry->d_inode) {
                         spin_unlock(&dentry->d_lock);
@@ -2734,19 +2798,43 @@ resume:
                         spin_acquire(&this_parent->d_lock.dep_map, 0, 1, _RET_IP_);
                         goto repeat;
                 }
-                dentry->d_count--;
+                if (!(dentry->d_flags & DCACHE_GENOCIDE)) {
+                        dentry->d_flags |= DCACHE_GENOCIDE;
+                        dentry->d_count--;
+                }
                 spin_unlock(&dentry->d_lock);
         }
         if (this_parent != root) {
-                next = this_parent->d_u.d_child.next;
-                this_parent->d_count--;
+                struct dentry *tmp;
+                struct dentry *child;
+
+                tmp = this_parent->d_parent;
+                if (!(this_parent->d_flags & DCACHE_GENOCIDE)) {
+                        this_parent->d_flags |= DCACHE_GENOCIDE;
+                        this_parent->d_count--;
+                }
+                rcu_read_lock();
                 spin_unlock(&this_parent->d_lock);
-                this_parent = this_parent->d_parent;
+                child = this_parent;
+                this_parent = tmp;
                 spin_lock(&this_parent->d_lock);
+                /* might go back up the wrong parent if we have had a rename
+                 * or deletion */
+                if (this_parent != child->d_parent ||
+                                read_seqretry(&rename_lock, seq)) {
+                        spin_unlock(&this_parent->d_lock);
+                        spin_unlock(&dcache_lock);
+                        rcu_read_unlock();
+                        goto rename_retry;
+                }
+                rcu_read_unlock();
+                next = child->d_u.d_child.next;
                 goto resume;
         }
         spin_unlock(&this_parent->d_lock);
         spin_unlock(&dcache_lock);
+        if (read_seqretry(&rename_lock, seq))
+                goto rename_retry;
 }
 
 /**