On Thu, Sep 05, 2013 at 01:32:10PM +0200, Miklos Szeredi wrote:
> On Thu, Sep 5, 2013 at 1:18 PM, Al Viro <v...@zeniv.linux.org.uk> wrote:
> > Something's really odd with locking here. You are take d_lock, do one
> > check, set flag, drop d_lock, grab rename_lock, do another check (taking
> > and dropping d_lock in process), and, in case that check fails, grab
> > d_lock again to clear the flag.
> >
> > At the very least it's a massive overkill. Just grab rename_lock, then
> > d_lock, then do the damn check and set the flag only on success. Moreover,
> > with rename_lock held, do you need d_lock on ancestors to mess with in
> > has_unlinked_ancestor()?
>
> Yes, we need hard exclusion for the __d_drop() part. rename_lock can
> provide one if we always take it for write in
> check_submounts_and_drop(). But if we only take it for read then
> that's not enough.
>
> And we do in fact also need DCACHE_MOUNTED set *before* checking
> ancestors. Otherwise check_submounts_and_drop() could succeed and
> has_unlinked_ancestor() return false, resulting in a dropped dentry
> and a mount below it. Though this is mostly theoretical at this
> point.
Maybe something like this. Has less ugly locking. Untested.
Thanks,
Miklos
---
fs/dcache.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
fs/internal.h | 1 +
fs/namespace.c | 11 +++++------
3 files changed, 55 insertions(+), 6 deletions(-)
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1159,6 +1159,55 @@ int have_submounts(struct dentry *parent
}
EXPORT_SYMBOL(have_submounts);
+static bool __has_unlinked_ancestor(struct dentry *dentry)
+{
+ struct dentry *this;
+
+ for (this = dentry; !IS_ROOT(this); this = this->d_parent) {
+ int is_unhashed;
+
+ /* Need exclusion wrt. check_submounts_and_drop() */
+ spin_lock(&this->d_lock);
+ is_unhashed = d_unhashed(this);
+ spin_unlock(&this->d_lock);
+
+ if (is_unhashed)
+ return true;
+ }
+ return false;
+}
+
+/*
+ * Called by mount code to check if the mountpoint is reachable (e.g. NFS can
+ * unhash a directory dentry and then the complete subtree can become
+ * unreachable).
+ */
+int d_set_mounted(struct dentry *dentry)
+{
+ int ret = 0;
+
+ write_seqlock(&rename_lock);
+ spin_lock(&dentry->d_lock);
+ dentry->d_flags |= DCACHE_MOUNTED;
+ if (!IS_ROOT(dentry)) {
+ ret = -ENOENT;
+ if (d_unhashed(dentry)) {
+ dentry->d_flags &= ~DCACHE_MOUNTED;
+ goto out;
+ }
+ spin_unlock(&dentry->d_lock);
+ if (__has_unlinked_ancestor(dentry->d_parent)) {
+ spin_lock(&dentry->d_lock);
+ dentry->d_flags &= ~DCACHE_MOUNTED;
+ spin_unlock(&dentry->d_lock);
+ }
+ ret = 0;
+ }
+out:
+ write_sequnlock(&rename_lock);
+ return ret;
+}
+
/*
* Search the dentry child list of the specified parent,
* and move any unused dentries to the end of the unused
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -126,6 +126,7 @@ extern int invalidate_inodes(struct supe
* dcache.c
*/
extern struct dentry *__d_alloc(struct super_block *, const struct qstr *);
+extern int d_set_mounted(struct dentry *dentry);
/*
* read_write.c
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -611,6 +611,7 @@ static struct mountpoint *new_mountpoint
{
struct list_head *chain = mountpoint_hashtable + hash(NULL, dentry);
struct mountpoint *mp;
+ int ret;
list_for_each_entry(mp, chain, m_hash) {
if (mp->m_dentry == dentry) {
@@ -626,14 +627,12 @@ static struct mountpoint *new_mountpoint
if (!mp)
return ERR_PTR(-ENOMEM);
- spin_lock(&dentry->d_lock);
- if (d_unlinked(dentry)) {
- spin_unlock(&dentry->d_lock);
+ ret = d_set_mounted(dentry);
+ if (ret) {
kfree(mp);
- return ERR_PTR(-ENOENT);
+ return ERR_PTR(ret);
}
- dentry->d_flags |= DCACHE_MOUNTED;
- spin_unlock(&dentry->d_lock);
+
mp->m_dentry = dentry;
mp->m_count = 1;
list_add(&mp->m_hash, chain);
--
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