[PATCH 08/53] VFS/xfs: drop parent lock across d_alloc_parallel() in d_add_ci()

[NeilBrown <neilb@ownmail.net>]

From: NeilBrown <neil@brown.name>

A proposed change will invert the lock ordering between
d_alloc_parallel() and inode_lock() on the parent.
When that happens it will not be safe to call d_alloc_parallel() while
holding the parent lock - even shared.

We don't need to keep the parent lock held when d_add_ci() is run - the
VFS doesn't need it as dentry is exclusively held due to
DCACHE_PAR_LOOKUP and the filesystem has finished its work.

So drop and reclaim the lock (shared or exclusive as determined by
LOOKUP_SHARED) to avoid future deadlock.

Signed-off-by: NeilBrown <neil@brown.name>
---
 fs/dcache.c            | 18 +++++++++++++++++-
 fs/xfs/xfs_iops.c      |  3 ++-
 include/linux/dcache.h |  3 ++-
 3 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index c12319097d6e..a1219b446b74 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2225,6 +2225,7 @@ EXPORT_SYMBOL(d_obtain_root);
  * @dentry: the negative dentry that was passed to the parent's lookup func
  * @inode:  the inode case-insensitive lookup has found
  * @name:   the case-exact name to be associated with the returned dentry
+ * @bool:   %true if lookup was performed with LOOKUP_SHARED
  *
  * This is to avoid filling the dcache with case-insensitive names to the
  * same inode, only the actual correct case is stored in the dcache for
@@ -2237,7 +2238,7 @@ EXPORT_SYMBOL(d_obtain_root);
  * the exact case, and return the spliced entry.
  */
 struct dentry *d_add_ci(struct dentry *dentry, struct inode *inode,
-			struct qstr *name)
+			struct qstr *name, bool shared)
 {
 	struct dentry *found, *res;
 
@@ -2250,6 +2251,17 @@ struct dentry *d_add_ci(struct dentry *dentry, struct inode *inode,
 		iput(inode);
 		return found;
 	}
+	/*
+	 * We are holding parent lock and so don't want to wait for a
+	 * d_in_lookup() dentry.  We can safely drop the parent lock and
+	 * reclaim it as we have exclusive access to dentry as it is
+	 * d_in_lookup() (so ->d_parent is stable) and we are near the
+	 * end ->lookup() and will shortly drop the lock anyway.
+	 */
+	if (shared)
+		inode_unlock_shared(d_inode(dentry->d_parent));
+	else
+		inode_unlock(d_inode(dentry->d_parent));
 	if (d_in_lookup(dentry)) {
 		found = d_alloc_parallel(dentry->d_parent, name);
 		if (IS_ERR(found) || !d_in_lookup(found)) {
@@ -2263,6 +2275,10 @@ struct dentry *d_add_ci(struct dentry *dentry, struct inode *inode,
 			return ERR_PTR(-ENOMEM);
 		}
 	}
+	if (shared)
+		inode_lock_shared(d_inode(dentry->d_parent));
+	else
+		inode_lock_nested(d_inode(dentry->d_parent), I_MUTEX_PARENT);
 	res = d_splice_alias(inode, found);
 	if (res) {
 		d_lookup_done(found);
diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index 208543e57eda..ec19d3ec7cf0 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -35,6 +35,7 @@
 #include <linux/security.h>
 #include <linux/iversion.h>
 #include <linux/fiemap.h>
+#include <linux/namei.h> // for LOOKUP_SHARED
 
 /*
  * Directories have different lock order w.r.t. mmap_lock compared to regular
@@ -369,7 +370,7 @@ xfs_vn_ci_lookup(
 	/* else case-insensitive match... */
 	dname.name = ci_name.name;
 	dname.len = ci_name.len;
-	dentry = d_add_ci(dentry, VFS_I(ip), &dname);
+	dentry = d_add_ci(dentry, VFS_I(ip), &dname, !!(flags & LOOKUP_SHARED));
 	kfree(ci_name.name);
 	return dentry;
 }
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 2a3ebd368ed9..a97eb151d9db 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -251,7 +251,8 @@ struct dentry *d_duplicate(struct dentry *dentry);
 /* weird procfs mess; *NOT* exported */
 extern struct dentry * d_splice_alias_ops(struct inode *, struct dentry *,
 					  const struct dentry_operations *);
-extern struct dentry * d_add_ci(struct dentry *, struct inode *, struct qstr *);
+extern struct dentry * d_add_ci(struct dentry *, struct inode *, struct qstr *,
+				bool);
 extern bool d_same_name(const struct dentry *dentry, const struct dentry *parent,
 			const struct qstr *name);
 extern struct dentry *d_find_any_alias(struct inode *inode);
-- 
2.50.0.107.gf914562f5916.dirty