autofs4 - fix get_next_positive_subdir() (a45440f0) · Commits · 戴 / test

Following a report of a crash during an automount expire I found that the locking in fs/autofs4/expire.c:get_next_positive_subdir() was wrong. Not only is the locking wrong but the function is more complex than it needs to be. The function is meant to calculate (and dget) the next entry in the list of directories contained in the root of an autofs mount point (an autofs indirect mount to be precise). The main problem was that the d_lock of the owner of the list was not being taken when walking the list, which lead to list corruption under load. The only other lock that needs to be taken is against the next dentry candidate so it can be checked for usability. Signed-off-by:

Ian Kent <raven@themaw.net> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

fs/autofs4/expire.c

+13 −18

Original line number	Diff line number	Diff line
		@@ -94,25 +94,21 @@ static struct dentry get_next_positive_subdir(struct dentry prev,
		{
		struct autofs_sb_info *sbi = autofs4_sbi(root->d_sb);
		struct list_head *next;
		struct dentry p, q;
		struct dentry *q;

		spin_lock(&sbi->lookup_lock);

		if (prev == NULL) {
		spin_lock(&root->d_lock);

		if (prev)
		next = prev->d_u.d_child.next;
		else {
		prev = dget_dlock(root);
		next = prev->d_subdirs.next;
		p = prev;
		goto start;
		}

		p = prev;
		spin_lock(&p->d_lock);
		again:
		next = p->d_u.d_child.next;
		start:
		cont:
		if (next == &root->d_subdirs) {
		spin_unlock(&p->d_lock);
		spin_unlock(&root->d_lock);
		spin_unlock(&sbi->lookup_lock);
		dput(prev);
		return NULL;
		@@ -121,16 +117,15 @@ start:
		q = list_entry(next, struct dentry, d_u.d_child);

		spin_lock_nested(&q->d_lock, DENTRY_D_LOCK_NESTED);
		/* Negative dentry - try next */
		if (!simple_positive(q)) {
		spin_unlock(&p->d_lock);
		lock_set_subclass(&q->d_lock.dep_map, 0, _RET_IP_);
		p = q;
		goto again;
		/* Already gone or negative dentry (under construction) - try next */
		if (q->d_count == 0 \|\| !simple_positive(q)) {
		spin_unlock(&q->d_lock);
		next = q->d_u.d_child.next;
		goto cont;
		}
		dget_dlock(q);
		spin_unlock(&q->d_lock);
		spin_unlock(&p->d_lock);
		spin_unlock(&root->d_lock);
		spin_unlock(&sbi->lookup_lock);

		dput(prev);

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