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Date: Thu, 8 Aug 2013 17:24:42 +0200
From: Miklos Szeredi <miklos@...redi.hu>
To: rwheeler@...hat.com, avati@...hat.com, viro@...IV.linux.org.uk
Cc: bfoster@...hat.com, dhowells@...hat.com, eparis@...hat.com,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
mszeredi@...e.cz, Steven Whitehouse <swhiteho@...hat.com>,
Trond Myklebust <Trond.Myklebust@...app.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: [PATCH 1/9] vfs: check submounts and drop atomically
From: Miklos Szeredi <mszeredi@...e.cz>
We check submounts before doing d_drop() on a non-empty directory dentry in
NFS (have_submounts()), but we do not exclude a racing mount.
Process A: have_submounts() -> returns false
Process B: mount() -> success
Process A: d_drop()
This patch prepares the ground for the fix by doing the following
operations all under the same rename lock:
have_submounts()
shrink_dcache_parent()
d_drop()
This is actually an optimization since have_submounts() and
shrink_dcache_parent() both traverse the same dentry tree separately.
Signed-off-by: Miklos Szeredi <mszeredi@...e.cz>
CC: David Howells <dhowells@...hat.com>
CC: Steven Whitehouse <swhiteho@...hat.com>
CC: Trond Myklebust <Trond.Myklebust@...app.com>
CC: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
fs/dcache.c | 157 +++++++++++++++++++++++++++++++++++++++++++++++++
include/linux/dcache.h | 1 +
2 files changed, 158 insertions(+)
diff --git a/fs/dcache.c b/fs/dcache.c
index 87bdb53..020004d 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1224,6 +1224,163 @@ void shrink_dcache_parent(struct dentry * parent)
}
EXPORT_SYMBOL(shrink_dcache_parent);
+static int __check_submounts_and_drop(struct dentry *parent,
+ struct list_head *dispose)
+{
+ struct dentry *this_parent;
+ struct list_head *next;
+ unsigned seq;
+ int found = 0;
+ int locked = 0;
+
+ seq = read_seqbegin(&rename_lock);
+again:
+ this_parent = parent;
+ spin_lock(&this_parent->d_lock);
+repeat:
+ next = this_parent->d_subdirs.next;
+resume:
+ while (next != &this_parent->d_subdirs) {
+ struct list_head *tmp = next;
+ struct dentry *dentry;
+
+ dentry = list_entry(tmp, struct dentry, d_u.d_child);
+ next = tmp->next;
+
+ spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
+ if (d_mountpoint(dentry)) {
+ spin_unlock(&dentry->d_lock);
+ found = -EBUSY;
+ goto out;
+ }
+
+ /*
+ * move only zero ref count dentries to the dispose list.
+ *
+ * Those which are presently on the shrink list, being processed
+ * by shrink_dentry_list(), shouldn't be moved. Otherwise the
+ * loop in shrink_dcache_parent() might not make any progress
+ * and loop forever.
+ */
+ if (dentry->d_count) {
+ dentry_lru_del(dentry);
+ } else if (!(dentry->d_flags & DCACHE_SHRINK_LIST)) {
+ dentry_lru_move_list(dentry, dispose);
+ dentry->d_flags |= DCACHE_SHRINK_LIST;
+ found++;
+ }
+ /*
+ * We can return to the caller if we have found some (this
+ * ensures forward progress). We'll be coming back to find
+ * the rest.
+ */
+ if (found && need_resched()) {
+ spin_unlock(&dentry->d_lock);
+ goto out;
+ }
+
+ /*
+ * Descend a level if the d_subdirs list is non-empty.
+ */
+ if (!list_empty(&dentry->d_subdirs)) {
+ spin_unlock(&this_parent->d_lock);
+ spin_release(&dentry->d_lock.dep_map, 1, _RET_IP_);
+ this_parent = dentry;
+ spin_acquire(&this_parent->d_lock.dep_map, 0, 1, _RET_IP_);
+ goto repeat;
+ }
+
+ spin_unlock(&dentry->d_lock);
+ }
+ /*
+ * All done at this level ... ascend and resume the search.
+ */
+ if (this_parent != parent) {
+ struct dentry *child = this_parent;
+ this_parent = try_to_ascend(this_parent, locked, seq);
+ if (!this_parent)
+ goto rename_retry;
+ next = child->d_u.d_child.next;
+ goto resume;
+ }
+ if (!locked && read_seqretry(&rename_lock, seq)) {
+ spin_unlock(&this_parent->d_lock);
+ goto rename_retry;
+ }
+ if (d_mountpoint(this_parent))
+ found = -EBUSY;
+ if (!found)
+ __d_drop(this_parent);
+out:
+ spin_unlock(&this_parent->d_lock);
+
+ if (locked)
+ write_sequnlock(&rename_lock);
+ return found;
+
+rename_retry:
+ if (found)
+ return found;
+ if (locked)
+ goto again;
+ locked = 1;
+ write_seqlock(&rename_lock);
+ goto again;
+}
+
+/**
+ * check_submounts_and_drop - prune dcache, check for submounts and drop
+ *
+ * All done as a single atomic operation relative to has_unlinked_ancestor().
+ * Returns 0 if successfully unhashed @parent. If there were submounts then
+ * return -EBUSY.
+ *
+ * @dentry: dentry to prune and drop
+ */
+int check_submounts_and_drop(struct dentry *dentry)
+{
+ int ret = 0;
+
+ /* Negative dentries can be dropped without further checks */
+ if (!dentry->d_inode) {
+ d_drop(dentry);
+ goto out;
+ }
+
+ spin_lock(&dentry->d_lock);
+ if (d_unhashed(dentry))
+ goto out_unlock;
+ if (list_empty(&dentry->d_subdirs)) {
+ if (d_mountpoint(dentry)) {
+ ret = -EBUSY;
+ goto out_unlock;
+ }
+ __d_drop(dentry);
+ goto out_unlock;
+ }
+ spin_unlock(&dentry->d_lock);
+
+ for (;;) {
+ LIST_HEAD(dispose);
+ ret = __check_submounts_and_drop(dentry, &dispose);
+ if (!list_empty(&dispose))
+ shrink_dentry_list(&dispose);
+
+ if (ret <= 0)
+ break;
+
+ cond_resched();
+ }
+
+out:
+ return ret;
+
+out_unlock:
+ spin_unlock(&dentry->d_lock);
+ goto out;
+}
+EXPORT_SYMBOL(check_submounts_and_drop);
+
/**
* __d_alloc - allocate a dcache entry
* @sb: filesystem it will belong to
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index b90337c..41b21ca 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -251,6 +251,7 @@ extern void d_prune_aliases(struct inode *);
/* test whether we have any submounts in a subdir tree */
extern int have_submounts(struct dentry *);
+extern int check_submounts_and_drop(struct dentry *);
/*
* This adds the entry to the hash queues.
--
1.8.1.4
--
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