author: Al Viro <viro@zeniv.linux.org.uk> 2019-09-15 12:12:39 -0400
committer: Al Viro <viro@zeniv.linux.org.uk> 2019-09-22 19:40:10 -0400
commit: d4f4de5e5ef8efde85febb6876cd3c8ab1631999
parent: 4d856f72c10ecb060868ed10ff1b1453943fc6c8
Commit Summary:
Diffstat:
1 file changed, 65 insertions, 63 deletions
diff --git a/fs/libfs.c b/fs/libfs.c
index c9b2850c0f7c..8e023b08a240 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -89,58 +89,47 @@ 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 passed to caller via *<res>.
+ * If no such element exists, the anchor of list is returned
+ * and *<res> is set to NULL.
+ */
+static struct list_head *scan_positives(struct dentry *cursor,
+ struct list_head *p,
+ loff_t count,
+ struct dentry **res)
{
- 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;
- break;
+ // 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(*res);
+ *res = found;
+ return p;
}
loff_t dcache_dir_lseek(struct file *file, loff_t offset, int whence)
@@ -158,17 +147,28 @@ loff_t dcache_dir_lseek(struct file *file, loff_t offset, int whence)
return -EINVAL;
}
if (offset != file->f_pos) {
+ struct dentry *cursor = file->private_data;
+ struct dentry *to = NULL;
+ struct list_head *p;
+
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;
-
- inode_lock_shared(dentry->d_inode);
- to = next_positive(dentry, &dentry->d_subdirs, n);
- move_cursor(cursor, to ? &to->d_child : NULL);
- inode_unlock_shared(dentry->d_inode);
+ inode_lock_shared(dentry->d_inode);
+
+ if (file->f_pos > 2) {
+ p = scan_positives(cursor, &dentry->d_subdirs,
+ file->f_pos - 2, &to);
+ spin_lock(&dentry->d_lock);
+ list_move(&cursor->d_child, p);
+ spin_unlock(&dentry->d_lock);
+ } else {
+ spin_lock(&dentry->d_lock);
+ list_del_init(&cursor->d_child);
+ spin_unlock(&dentry->d_lock);
}
+
+ dput(to);
+
+ inode_unlock_shared(dentry->d_inode);
}
return offset;
}
@@ -190,25 +190,29 @@ 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
+ p = &cursor->d_child;
+
+ while ((p = scan_positives(cursor, p, 1, &next)) != anchor) {
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++;
}
- if (moved)
- move_cursor(cursor, p);
+ spin_lock(&dentry->d_lock);
+ list_move_tail(&cursor->d_child, p);
+ spin_unlock(&dentry->d_lock);
+ dput(next);
+
return 0;
}
EXPORT_SYMBOL(dcache_readdir);