autofs4: avoid taking fs_lock during rcu-walk

->fs_lock protects AUTOFS_INF_EXPIRING.  We need to be sure that once
the flag is set, no new references beneath the dentry are taken.  So
rcu-walk currently needs to take fs_lock before checking the flag.  This
hurts performance.

Change the expiry to a two-stage process.  First set AUTOFS_INF_NO_RCU
which forces any path walk into ref-walk mode, then drop the lock and
call synchronize_rcu().  Once that returns we can be sure no rcu-walk is
active beneath the dentry and we can check reference counts again.

Now during an RCU-walk we can test AUTOFS_INF_EXPIRING without taking
the lock as along as we test AUTOFS_INF_NO_RCU too.  If either are set,
we must abort the RCU-walk If neither are set, we know that refcounts
will be tested again after we finish the RCU-walk so we are safe to
continue.

->fs_lock is still taken in d_manage() to check for a non-trap
directory.  That will be resolved in the next patch.
Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Ian Kent <raven@themaw.net>
Tested-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/autofs4/autofs_i.h

@@ -79,6 +79,10 @@ struct autofs_info {
 };
 
 #define AUTOFS_INF_EXPIRING	(1<<0) /* dentry is in the process of expiring */
+#define AUTOFS_INF_NO_RCU	(1<<1) /* the dentry is being considered
+					* for expiry, so RCU_walk is
+					* not permitted
+					*/
 #define AUTOFS_INF_PENDING	(1<<2) /* dentry pending mount */
 
 struct autofs_wait_queue {

fs/autofs4/expire.c

@@ -320,12 +320,21 @@ struct dentry *autofs4_expire_direct(struct super_block *sb,
 	/* No point expiring a pending mount */
 	if (ino->flags & AUTOFS_INF_PENDING)
 		goto out;
+	if (!autofs4_direct_busy(mnt, root, timeout, do_now)) {
+		ino->flags |= AUTOFS_INF_NO_RCU;
+		spin_unlock(&sbi->fs_lock);
+		synchronize_rcu();
+		spin_lock(&sbi->fs_lock);
 		if (!autofs4_direct_busy(mnt, root, timeout, do_now)) {
 			ino->flags |= AUTOFS_INF_EXPIRING;
+			smp_mb();
+			ino->flags &= ~AUTOFS_INF_NO_RCU;
 			init_completion(&ino->expire_complete);
 			spin_unlock(&sbi->fs_lock);
 			return root;
 		}
+		ino->flags &= ~AUTOFS_INF_NO_RCU;
+	}
 out:
 	spin_unlock(&sbi->fs_lock);
 	dput(root);
@@ -442,12 +451,29 @@ struct dentry *autofs4_expire_indirect(struct super_block *sb,
 	dentry = NULL;
 	while ((dentry = get_next_positive_subdir(dentry, root))) {
 		spin_lock(&sbi->fs_lock);
+		ino = autofs4_dentry_ino(dentry);
+		if (ino->flags & AUTOFS_INF_NO_RCU)
+			expired = NULL;
+		else
 			expired = should_expire(dentry, mnt, timeout, how);
-		if (expired) {
+		if (!expired) {
+			spin_unlock(&sbi->fs_lock);
+			continue;
+		}
+		ino = autofs4_dentry_ino(expired);
+		ino->flags |= AUTOFS_INF_NO_RCU;
+		spin_unlock(&sbi->fs_lock);
+		synchronize_rcu();
+		spin_lock(&sbi->fs_lock);
+		if (should_expire(expired, mnt, timeout, how)) {
 			if (expired != dentry)
 				dput(dentry);
 			goto found;
 		}
+
+		ino->flags &= ~AUTOFS_INF_NO_RCU;
+		if (expired != dentry)
+			dput(expired);
 		spin_unlock(&sbi->fs_lock);
 	}
 	return NULL;
@@ -455,8 +481,9 @@ struct dentry *autofs4_expire_indirect(struct super_block *sb,
 found:
 	DPRINTK("returning %p %.*s",
 		expired, (int)expired->d_name.len, expired->d_name.name);
-	ino = autofs4_dentry_ino(expired);
 	ino->flags |= AUTOFS_INF_EXPIRING;
+	smp_mb();
+	ino->flags &= ~AUTOFS_INF_NO_RCU;
 	init_completion(&ino->expire_complete);
 	spin_unlock(&sbi->fs_lock);
 	spin_lock(&sbi->lookup_lock);
@@ -476,11 +503,14 @@ int autofs4_expire_wait(struct dentry *dentry, int rcu_walk)
 	int status;
 
 	/* Block on any pending expire */
+	if (!(ino->flags & (AUTOFS_INF_EXPIRING | AUTOFS_INF_NO_RCU)))
+		return 0;
+	if (rcu_walk)
+		return -ECHILD;
+
 	spin_lock(&sbi->fs_lock);
 	if (ino->flags & AUTOFS_INF_EXPIRING) {
 		spin_unlock(&sbi->fs_lock);
-		if (rcu_walk)
-			return -ECHILD;
 
 		DPRINTK("waiting for expire %p name=%.*s",
 			 dentry, dentry->d_name.len, dentry->d_name.name);