[PATCH v2 4/5] afs: lookup_atsys to drop and reclaim lock.

[NeilBrown]

From: NeilBrown <neil at brown.name>

If afs is asked to lookup a name ending with @sys, it needs to look up
a different name for which it might need to allocate a dentry with
d_alloc_parallel().
This is done while the parent lock is held which will be a problem in
a future patch where the ordering of the parent lock and
d_alloc_parallel() locking is reversed.

There is no actual need to hold the lock during this nested lookup, so
with this patch we drop the lock and reclaim it, and use
lookup_noperm_unlocked().

Signed-off-by: NeilBrown <neil at brown.name>
---
 fs/afs/dir.c | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/fs/afs/dir.c b/fs/afs/dir.c
index 1edaa7323428..b8773adc4e3e 100644
--- a/fs/afs/dir.c
+++ b/fs/afs/dir.c
@@ -911,7 +911,8 @@ static struct inode *afs_do_lookup(struct inode *dir, struct dentry *dentry)
 /*
  * Look up an entry in a directory with @sys substitution.
  */
-static struct dentry *afs_lookup_atsys(struct inode *dir, struct dentry *dentry)
+static struct dentry *afs_lookup_atsys(struct inode *dir, struct dentry *dentry,
+				       unsigned int flags)
 {
 	struct afs_sysnames *subs;
 	struct afs_net *net = afs_i2net(dir);
@@ -936,6 +937,15 @@ static struct dentry *afs_lookup_atsys(struct inode *dir, struct dentry *dentry)
 	refcount_inc(&subs->usage);
 	read_unlock(&net->sysnames_lock);
 
+	/*
+	 * We need the directory to be unlocked so we can perform other lookups.
+	 * We don't really need the lock any more. The in-lookup status of
+	 * dentry gives us sufficient exclusion.
+	 */
+	if (flags & LOOKUP_SHARED)
+		inode_unlock_shared(dir);
+	else
+		inode_unlock(dir);
 	for (i = 0; i < subs->nr; i++) {
 		name = subs->subs[i];
 		len = dentry->d_name.len - 4 + strlen(name);
@@ -945,7 +955,7 @@ static struct dentry *afs_lookup_atsys(struct inode *dir, struct dentry *dentry)
 		}
 
 		strcpy(p, name);
-		ret = lookup_noperm(&QSTR(buf), dentry->d_parent);
+		ret = lookup_noperm_unlocked(&QSTR(buf), dentry->d_parent);
 		if (IS_ERR(ret) || d_is_positive(ret))
 			goto out_s;
 		dput(ret);
@@ -956,6 +966,15 @@ static struct dentry *afs_lookup_atsys(struct inode *dir, struct dentry *dentry)
 	 */
 	ret = NULL;
 out_s:
+	/*
+	 * Cannot take parent lock while we hold an in-lookup dentry so
+	 * must make sure that lookup is done.
+	 */
+	d_lookup_done(dentry);
+	if (flags & LOOKUP_SHARED)
+		inode_lock_shared(dir);
+	else
+		inode_lock_nested(dir, I_MUTEX_PARENT);
 	afs_put_sysnames(subs);
 	kfree(buf);
 out_p:
@@ -1001,7 +1020,7 @@ struct dentry *afs_lookup(struct inode *dir, struct dentry *dentry,
 	    dentry->d_name.name[dentry->d_name.len - 3] == 's' &&
 	    dentry->d_name.name[dentry->d_name.len - 2] == 'y' &&
 	    dentry->d_name.name[dentry->d_name.len - 1] == 's')
-		return afs_lookup_atsys(dir, dentry);
+		return afs_lookup_atsys(dir, dentry, flags);
 
 	afs_stat_v(dvnode, n_lookup);
 	inode = afs_do_lookup(dir, dentry);
-- 
2.50.0.107.gf914562f5916.dirty