autofs4: allow RCU-walk to walk through autofs4

This series teaches autofs about RCU-walk so that we don't drop straight
into REF-walk when we hit an autofs directory, and so that we avoid
spinlocks as much as possible when performing an RCU-walk.

This is needed so that the benefits of the recent NFS support for
RCU-walk are fully available when NFS filesystems are automounted.

Patches have been carefully reviewed and tested both with test suites
and in production - thanks a lot to Ian Kent for his support there.

This patch (of 6):

Any attempt to look up a pathname that passes though an autofs4 mount is
currently forced out of RCU-walk into REF-walk.

This can significantly hurt performance of many-thread work loads on
many-core systems, especially if the automounted filesystem supports
RCU-walk but doesn't get to benefit from it.

So if autofs4_d_manage is called with rcu_walk set, only fail with -ECHILD
if it is necessary to wait longer than a spinlock.

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>

4 files changed, 34 insertions, 18 deletions

diff --git a/fs/autofs4/autofs_i.h b/fs/autofs4/autofs_i.h
index 9e359fb20c0a..2f1032f12d91 100644
--- a/fs/autofs4/autofs_i.h
+++ b/fs/autofs4/autofs_i.h
@@ -148,7 +148,7 @@ void autofs4_free_ino(struct autofs_info *);
 
 /* Expiration */
 int is_autofs4_dentry(struct dentry *);
-int autofs4_expire_wait(struct dentry *dentry);
+int autofs4_expire_wait(struct dentry *dentry, int rcu_walk);
 int autofs4_expire_run(struct super_block *, struct vfsmount *,
                         struct autofs_sb_info *,
                         struct autofs_packet_expire __user *);
diff --git a/fs/autofs4/dev-ioctl.c b/fs/autofs4/dev-ioctl.c
index 5b570b6efa28..aaf96cb25452 100644
--- a/fs/autofs4/dev-ioctl.c
+++ b/fs/autofs4/dev-ioctl.c
@@ -450,7 +450,7 @@ static int autofs_dev_ioctl_requester(struct file *fp,
         ino = autofs4_dentry_ino(path.dentry);
         if (ino) {
                 err = 0;
-                autofs4_expire_wait(path.dentry);
+                autofs4_expire_wait(path.dentry, 0);
                 spin_lock(&sbi->fs_lock);
                 param->requester.uid = from_kuid_munged(current_user_ns(), ino->uid);
                 param->requester.gid = from_kgid_munged(current_user_ns(), ino->gid);
diff --git a/fs/autofs4/expire.c b/fs/autofs4/expire.c
index 8fa3895cda02..1695eda8ac18 100644
--- a/fs/autofs4/expire.c
+++ b/fs/autofs4/expire.c
@@ -461,7 +461,7 @@ found:
         return expired;
 }
 
-int autofs4_expire_wait(struct dentry *dentry)
+int autofs4_expire_wait(struct dentry *dentry, int rcu_walk)
 {
         struct autofs_sb_info *sbi = autofs4_sbi(dentry->d_sb);
         struct autofs_info *ino = autofs4_dentry_ino(dentry);
@@ -471,6 +471,8 @@ int autofs4_expire_wait(struct dentry *dentry)
         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);
diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
index cdb25ebccc4c..2296c8301b66 100644
--- a/fs/autofs4/root.c
+++ b/fs/autofs4/root.c
@@ -210,7 +210,8 @@ next:
         return NULL;
 }
 
-static struct dentry *autofs4_lookup_expiring(struct dentry *dentry)
+static struct dentry *autofs4_lookup_expiring(struct dentry *dentry,
+                                              bool rcu_walk)
 {
         struct autofs_sb_info *sbi = autofs4_sbi(dentry->d_sb);
         struct dentry *parent = dentry->d_parent;
@@ -229,6 +230,11 @@ static struct dentry *autofs4_lookup_expiring(struct dentry *dentry)
                 struct dentry *expiring;
                 struct qstr *qstr;
 
+                if (rcu_walk) {
+                        spin_unlock(&sbi->lookup_lock);
+                        return ERR_PTR(-ECHILD);
+                }
+
                 ino = list_entry(p, struct autofs_info, expiring);
                 expiring = ino->dentry;
 
@@ -264,13 +270,15 @@ next:
         return NULL;
 }
 
-static int autofs4_mount_wait(struct dentry *dentry)
+static int autofs4_mount_wait(struct dentry *dentry, bool rcu_walk)
 {
         struct autofs_sb_info *sbi = autofs4_sbi(dentry->d_sb);
         struct autofs_info *ino = autofs4_dentry_ino(dentry);
         int status = 0;
 
         if (ino->flags & AUTOFS_INF_PENDING) {
+                if (rcu_walk)
+                        return -ECHILD;
                 DPRINTK("waiting for mount name=%.*s",
                         dentry->d_name.len, dentry->d_name.name);
                 status = autofs4_wait(sbi, dentry, NFY_MOUNT);
@@ -280,20 +288,22 @@ static int autofs4_mount_wait(struct dentry *dentry)
         return status;
 }
 
-static int do_expire_wait(struct dentry *dentry)
+static int do_expire_wait(struct dentry *dentry, bool rcu_walk)
 {
         struct dentry *expiring;
 
-        expiring = autofs4_lookup_expiring(dentry);
+        expiring = autofs4_lookup_expiring(dentry, rcu_walk);
+        if (IS_ERR(expiring))
+                return PTR_ERR(expiring);
         if (!expiring)
-                return autofs4_expire_wait(dentry);
+                return autofs4_expire_wait(dentry, rcu_walk);
         else {
                 /*
                  * If we are racing with expire the request might not
                  * be quite complete, but the directory has been removed
                  * so it must have been successful, just wait for it.
                  */
-                autofs4_expire_wait(expiring);
+                autofs4_expire_wait(expiring, 0);
                 autofs4_del_expiring(expiring);
                 dput(expiring);
         }
@@ -345,7 +355,7 @@ static struct vfsmount *autofs4_d_automount(struct path *path)
          * and the directory was removed, so just go ahead and try
          * the mount.
          */
-        status = do_expire_wait(dentry);
+        status = do_expire_wait(dentry, 0);
         if (status && status != -EAGAIN)
                 return NULL;
 
@@ -353,7 +363,7 @@ static struct vfsmount *autofs4_d_automount(struct path *path)
         spin_lock(&sbi->fs_lock);
         if (ino->flags & AUTOFS_INF_PENDING) {
                 spin_unlock(&sbi->fs_lock);
-                status = autofs4_mount_wait(dentry);
+                status = autofs4_mount_wait(dentry, 0);
                 if (status)
                         return ERR_PTR(status);
                 goto done;
@@ -394,7 +404,7 @@ static struct vfsmount *autofs4_d_automount(struct path *path)
                 }
                 ino->flags |= AUTOFS_INF_PENDING;
                 spin_unlock(&sbi->fs_lock);
-                status = autofs4_mount_wait(dentry);
+                status = autofs4_mount_wait(dentry, 0);
                 spin_lock(&sbi->fs_lock);
                 ino->flags &= ~AUTOFS_INF_PENDING;
                 if (status) {
@@ -430,21 +440,25 @@ static int autofs4_d_manage(struct dentry *dentry, bool rcu_walk)
                 return 0;
         }
 
-        /* We need to sleep, so we need pathwalk to be in ref-mode */
-        if (rcu_walk)
-                return -ECHILD;
-
         /* Wait for pending expires */
-        do_expire_wait(dentry);
+        if (do_expire_wait(dentry, rcu_walk) == -ECHILD)
+                return -ECHILD;
 
         /*
          * This dentry may be under construction so wait on mount
          * completion.
          */
-        status = autofs4_mount_wait(dentry);
+        status = autofs4_mount_wait(dentry, rcu_walk);
         if (status)
                 return status;
 
+        if (rcu_walk)
+                /* it is always safe to return 0 as the worst that
+                 * will happen is we retry in REF-walk mode.
+                 * Better than always taking a lock.
+                 */
+                return 0;
+
         spin_lock(&sbi->fs_lock);
         /*
          * If the dentry has been selected for expire while we slept