path: root/fs/autofs4/root.c
diff options
authorIan Kent <raven@themaw.net>2007-08-22 14:01:54 -0700
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2007-08-22 19:52:46 -0700
commit1864f7bd58351732593def024e73eca1f75bc352 (patch)
treebe7f048f3a41b257ece24805aa96453b81c78349 /fs/autofs4/root.c
parentf4768ffd1d4b7b07ae2c4c3d93c9f99cd68e996c (diff)
autofs4: deadlock during create
Due to inconsistent locking in the VFS between calls to lookup and revalidate deadlock can occur in the automounter. The inconsistency is that the directory inode mutex is held for both lookup and revalidate calls when called via lookup_hash whereas it is held only for lookup during a path walk. Consequently, if the mutex is held during a call to revalidate autofs4 can't release the mutex to callback the daemon as it can't know whether it owns the mutex. This situation happens when a process tries to create a directory within an automount and a second process also tries to create the same directory between the lookup and the mkdir. Since the first process has dropped the mutex for the daemon callback, the second process takes it during revalidate leading to deadlock between the autofs daemon and the second process when the daemon tries to create the mount point directory. After spending quite a bit of time trying to resolve this on more than one occassion, using rather complex and ulgy approaches, it turns out that just delaying the hashing of the dentry until the create operation works fine. Signed-off-by: Ian Kent <raven@themaw.net> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'fs/autofs4/root.c')
1 files changed, 17 insertions, 14 deletions
diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
index 2d4c8a3e604..45ff3d63b75 100644
--- a/fs/autofs4/root.c
+++ b/fs/autofs4/root.c
@@ -587,19 +587,20 @@ static struct dentry *autofs4_lookup(struct inode *dir, struct dentry *dentry, s
unhashed = autofs4_lookup_unhashed(sbi, dentry->d_parent, &dentry->d_name);
if (!unhashed) {
- * Mark the dentry incomplete, but add it. This is needed so
- * that the VFS layer knows about the dentry, and we can count
- * on catching any lookups through the revalidate.
- *
- * Let all the hard work be done by the revalidate function that
- * needs to be able to do this anyway..
- *
- * We need to do this before we release the directory semaphore.
+ * Mark the dentry incomplete but don't hash it. We do this
+ * to serialize our inode creation operations (symlink and
+ * mkdir) which prevents deadlock during the callback to
+ * the daemon. Subsequent user space lookups for the same
+ * dentry are placed on the wait queue while the daemon
+ * itself is allowed passage unresticted so the create
+ * operation itself can then hash the dentry. Finally,
+ * we check for the hashed dentry and return the newly
+ * hashed dentry.
dentry->d_op = &autofs4_root_dentry_operations;
dentry->d_fsdata = NULL;
- d_add(dentry, NULL);
+ d_instantiate(dentry, NULL);
} else {
struct autofs_info *ino = autofs4_dentry_ino(unhashed);
DPRINTK("rehash %p with %p", dentry, unhashed);
@@ -607,15 +608,17 @@ static struct dentry *autofs4_lookup(struct inode *dir, struct dentry *dentry, s
* If we are racing with expire the request might not
* be quite complete but the directory has been removed
* so it must have been successful, so just wait for it.
+ * We need to ensure the AUTOFS_INF_EXPIRING flag is clear
+ * before continuing as revalidate may fail when calling
+ * try_to_fill_dentry (returning EAGAIN) if we don't.
- if (ino && (ino->flags & AUTOFS_INF_EXPIRING)) {
+ while (ino && (ino->flags & AUTOFS_INF_EXPIRING)) {
DPRINTK("wait for incomplete expire %p name=%.*s",
unhashed, unhashed->d_name.len,
autofs4_wait(sbi, unhashed, NFY_NONE);
DPRINTK("request completed");
- d_rehash(unhashed);
dentry = unhashed;
@@ -658,7 +661,7 @@ static struct dentry *autofs4_lookup(struct inode *dir, struct dentry *dentry, s
* for all system calls, but it should be OK for the operations
* we permit from an autofs.
- if (dentry->d_inode && d_unhashed(dentry)) {
+ if (!oz_mode && d_unhashed(dentry)) {
* A user space application can (and has done in the past)
* remove and re-create this directory during the callback.
@@ -716,7 +719,7 @@ static int autofs4_dir_symlink(struct inode *dir,
strcpy(cp, symname);
inode = autofs4_get_inode(dir->i_sb, ino);
- d_instantiate(dentry, inode);
+ d_add(dentry, inode);
if (dir == dir->i_sb->s_root->d_inode)
dentry->d_op = &autofs4_root_dentry_operations;
@@ -844,7 +847,7 @@ static int autofs4_dir_mkdir(struct inode *dir, struct dentry *dentry, int mode)
return -ENOSPC;
inode = autofs4_get_inode(dir->i_sb, ino);
- d_instantiate(dentry, inode);
+ d_add(dentry, inode);
if (dir == dir->i_sb->s_root->d_inode)
dentry->d_op = &autofs4_root_dentry_operations;