Commit ad338d05 authored by Linus Torvalds's avatar Linus Torvalds
Browse files

Merge branch 'work.dcache' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 

Pull dcache_readdir() fixes from Al Viro:
 "The couple of patches you'd been OK with; no hlist conversion yet, and
  cursors are still in the list of children"

[ Al is referring to future work to avoid some nasty O(n**2) behavior
  with the readdir cursors when you have lots of concurrent readdirs.

  This is just a fix for a race with a trivial cleanup   - Linus ]

* 'work.dcache' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs:
  libfs: take cursors out of list when moving past the end of directory
  Fix the locking in dcache_readdir() and friends
parents 015c21ba 26b6c984

fs/libfs.c

+71 −66
Original line number Diff line number Diff line
@@ -89,58 +89,45 @@ int dcache_dir_close(struct inode *inode, struct file *file) 
EXPORT_SYMBOL(dcache_dir_close);



/* parent is locked at least shared */

static struct dentry *next_positive(struct dentry *parent,

				    struct list_head *from,

				    int count)

/*

 * Returns an element of siblings' list.

 * We are looking for <count>th positive after <p>; if

 * found, dentry is grabbed and returned to caller.

 * If no such element exists, NULL is returned.

 */

static struct dentry *scan_positives(struct dentry *cursor,

					struct list_head *p,

					loff_t count,

					struct dentry *last)

{

	unsigned *seq = &parent->d_inode->i_dir_seq, n;

	struct dentry *res;

	struct list_head *p;

	bool skipped;

	int i;

	struct dentry *dentry = cursor->d_parent, *found = NULL;



retry:

	i = count;

	skipped = false;

	n = smp_load_acquire(seq) & ~1;

	res = NULL;

	rcu_read_lock();

	for (p = from->next; p != &parent->d_subdirs; p = p->next) {

	spin_lock(&dentry->d_lock);

	while ((p = p->next) != &dentry->d_subdirs) {

		struct dentry *d = list_entry(p, struct dentry, d_child);

		if (!simple_positive(d)) {

			skipped = true;

		} else if (!--i) {

			res = d;

		// we must at least skip cursors, to avoid livelocks

		if (d->d_flags & DCACHE_DENTRY_CURSOR)

			continue;

		if (simple_positive(d) && !--count) {

			spin_lock_nested(&d->d_lock, DENTRY_D_LOCK_NESTED);

			if (simple_positive(d))

				found = dget_dlock(d);

			spin_unlock(&d->d_lock);

			if (likely(found))

				break;

			count = 1;

		}

		if (need_resched()) {

			list_move(&cursor->d_child, p);

			p = &cursor->d_child;

			spin_unlock(&dentry->d_lock);

			cond_resched();

			spin_lock(&dentry->d_lock);

		}

	rcu_read_unlock();

	if (skipped) {

		smp_rmb();

		if (unlikely(*seq != n))

			goto retry;

	}

	return res;

}



static void move_cursor(struct dentry *cursor, struct list_head *after)

{

	struct dentry *parent = cursor->d_parent;

	unsigned n, *seq = &parent->d_inode->i_dir_seq;

	spin_lock(&parent->d_lock);

	for (;;) {

		n = *seq;

		if (!(n & 1) && cmpxchg(seq, n, n + 1) == n)

			break;

		cpu_relax();

	}

	__list_del(cursor->d_child.prev, cursor->d_child.next);

	if (after)

		list_add(&cursor->d_child, after);

	else

		list_add_tail(&cursor->d_child, &parent->d_subdirs);

	smp_store_release(seq, n + 2);

	spin_unlock(&parent->d_lock);

	spin_unlock(&dentry->d_lock);

	dput(last);

	return found;

}



loff_t dcache_dir_lseek(struct file *file, loff_t offset, int whence)
@@ -158,18 +145,26 @@ loff_t dcache_dir_lseek(struct file *file, loff_t offset, int whence) 
			return -EINVAL;

	}

	if (offset != file->f_pos) {

		file->f_pos = offset;

		if (file->f_pos >= 2) {

		struct dentry *cursor = file->private_data;

			struct dentry *to;

			loff_t n = file->f_pos - 2;

		struct dentry *to = NULL;



		inode_lock_shared(dentry->d_inode);

			to = next_positive(dentry, &dentry->d_subdirs, n);

			move_cursor(cursor, to ? &to->d_child : NULL);



		if (offset > 2)

			to = scan_positives(cursor, &dentry->d_subdirs,

					    offset - 2, NULL);

		spin_lock(&dentry->d_lock);

		if (to)

			list_move(&cursor->d_child, &to->d_child);

		else

			list_del_init(&cursor->d_child);

		spin_unlock(&dentry->d_lock);

		dput(to);



		file->f_pos = offset;



		inode_unlock_shared(dentry->d_inode);

	}

	}

	return offset;

}

EXPORT_SYMBOL(dcache_dir_lseek);
@@ -190,25 +185,35 @@ int dcache_readdir(struct file *file, struct dir_context *ctx) 
{

	struct dentry *dentry = file->f_path.dentry;

	struct dentry *cursor = file->private_data;

	struct list_head *p = &cursor->d_child;

	struct dentry *next;

	bool moved = false;

	struct list_head *anchor = &dentry->d_subdirs;

	struct dentry *next = NULL;

	struct list_head *p;



	if (!dir_emit_dots(file, ctx))

		return 0;



	if (ctx->pos == 2)

		p = &dentry->d_subdirs;

	while ((next = next_positive(dentry, p, 1)) != NULL) {

		p = anchor;

	else if (!list_empty(&cursor->d_child))

		p = &cursor->d_child;

	else

		return 0;



	while ((next = scan_positives(cursor, p, 1, next)) != NULL) {

		if (!dir_emit(ctx, next->d_name.name, next->d_name.len,

			      d_inode(next)->i_ino, dt_type(d_inode(next))))

			break;

		moved = true;

		p = &next->d_child;

		ctx->pos++;

		p = &next->d_child;

	}

	if (moved)

		move_cursor(cursor, p);

	spin_lock(&dentry->d_lock);

	if (next)

		list_move_tail(&cursor->d_child, &next->d_child);

	else

		list_del_init(&cursor->d_child);

	spin_unlock(&dentry->d_lock);

	dput(next);



	return 0;

}

EXPORT_SYMBOL(dcache_readdir);

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