autofs4: fix direct mount pending expire race

For direct and offset type mounts that are covered by another mount we
cannot check the AUTOFS_INF_EXPIRING flag during a path walk which leads
to lookups walking into an expiring mount while it is being expired.

For example, for the direct multi-mount map entry with a couple of
offsets:

/race/mm1  /      <server1>:/<path1>
           /om1   <server2>:/<path2>
           /om2   <server1>:/<path3>

an autofs trigger mount is mounted on /race/mm1 and when accessed it is
over mounted and trigger mounts made for /race/mm1/om1 and /race/mm1/om2.
So it isn't possible for path walks to see the expiring flag at all and
they happily walk into the file system while it is expiring.

When expiring these mounts follow_down() must stop at the autofs mount and
all processes must block in the ->follow_link() method (except the daemon)
until the expire is complete.  This is done by decrementing the d_mounted
field of the autofs trigger mount root dentry until the expire is
completed.  In ->follow_link() all processes wait on the expire and the
mount following is completed for the daemon until the expire is complete.

Signed-off-by: Ian Kent <raven@themaw.net>
Cc: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 65 insertions, 26 deletions

diff --git a/fs/autofs4/autofs_i.h b/fs/autofs4/autofs_i.h
index 5d90ed3b4b43..4b40cbc71e9b 100644
--- a/fs/autofs4/autofs_i.h
+++ b/fs/autofs4/autofs_i.h
@@ -52,6 +52,8 @@ struct autofs_info {
 
         int             flags;
 
+        struct completion expire_complete;
+
         struct list_head active;
         struct list_head expiring;
 
@@ -69,6 +71,7 @@ struct autofs_info {
 };
 
 #define AUTOFS_INF_EXPIRING     (1<<0) /* dentry is in the process of expiring */
+#define AUTOFS_INF_MOUNTPOINT   (1<<1) /* mountpoint status for direct expire */
 
 struct autofs_wait_queue {
         wait_queue_head_t queue;
diff --git a/fs/autofs4/expire.c b/fs/autofs4/expire.c
index 19f5bea2704f..705b9f057fb3 100644
--- a/fs/autofs4/expire.c
+++ b/fs/autofs4/expire.c
@@ -259,13 +259,15 @@ static struct dentry *autofs4_expire_direct(struct super_block *sb,
         now = jiffies;
         timeout = sbi->exp_timeout;
 
-        /* Lock the tree as we must expire as a whole */
         spin_lock(&sbi->fs_lock);
         if (!autofs4_direct_busy(mnt, root, timeout, do_now)) {
                 struct autofs_info *ino = autofs4_dentry_ino(root);
-
-                /* Set this flag early to catch sys_chdir and the like */
+                if (d_mountpoint(root)) {
+                        ino->flags |= AUTOFS_INF_MOUNTPOINT;
+                        root->d_mounted--;
+                }
                 ino->flags |= AUTOFS_INF_EXPIRING;
+                init_completion(&ino->expire_complete);
                 spin_unlock(&sbi->fs_lock);
                 return root;
         }
@@ -392,6 +394,7 @@ found:
                 expired, (int)expired->d_name.len, expired->d_name.name);
         ino = autofs4_dentry_ino(expired);
         ino->flags |= AUTOFS_INF_EXPIRING;
+        init_completion(&ino->expire_complete);
         spin_unlock(&sbi->fs_lock);
         spin_lock(&dcache_lock);
         list_move(&expired->d_parent->d_subdirs, &expired->d_u.d_child);
@@ -429,6 +432,7 @@ int autofs4_expire_run(struct super_block *sb,
         spin_lock(&sbi->fs_lock);
         ino = autofs4_dentry_ino(dentry);
         ino->flags &= ~AUTOFS_INF_EXPIRING;
+        complete_all(&ino->expire_complete);
         spin_unlock(&sbi->fs_lock);
 
         return ret;
@@ -457,8 +461,14 @@ int autofs4_expire_multi(struct super_block *sb, struct vfsmount *mnt,
                 /* This is synchronous because it makes the daemon a
                    little easier */
                 ret = autofs4_wait(sbi, dentry, NFY_EXPIRE);
+
                 spin_lock(&sbi->fs_lock);
+                if (ino->flags & AUTOFS_INF_MOUNTPOINT) {
+                        sb->s_root->d_mounted++;
+                        ino->flags &= ~AUTOFS_INF_MOUNTPOINT;
+                }
                 ino->flags &= ~AUTOFS_INF_EXPIRING;
+                complete_all(&ino->expire_complete);
                 spin_unlock(&sbi->fs_lock);
                 dput(dentry);
         }
diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
index 1c2579de1f2e..adbd8559e870 100644
--- a/fs/autofs4/root.c
+++ b/fs/autofs4/root.c
@@ -141,6 +141,7 @@ static int try_to_fill_dentry(struct dentry *dentry, int flags)
                          dentry, dentry->d_name.len, dentry->d_name.name);
 
                 status = autofs4_wait(sbi, dentry, NFY_NONE);
+                wait_for_completion(&ino->expire_complete);
 
                 DPRINTK("expire done status=%d", status);
 
@@ -227,14 +228,32 @@ static void *autofs4_follow_link(struct dentry *dentry, struct nameidata *nd)
         DPRINTK("dentry=%p %.*s oz_mode=%d nd->flags=%d",
                 dentry, dentry->d_name.len, dentry->d_name.name, oz_mode,
                 nd->flags);
-
-        /* If it's our master or we shouldn't trigger a mount we're done */
-        lookup_type = nd->flags & (TRIGGER_FLAGS | TRIGGER_INTENTS);
-        if (oz_mode ||
-            !(lookup_type || dentry->d_flags & DCACHE_AUTOFS_PENDING))
+        /*
+         * For an expire of a covered direct or offset mount we need
+         * to beeak out of follow_down() at the autofs mount trigger
+         * (d_mounted--), so we can see the expiring flag, and manage
+         * the blocking and following here until the expire is completed.
+         */
+        if (oz_mode) {
+                spin_lock(&sbi->fs_lock);
+                if (ino->flags & AUTOFS_INF_EXPIRING) {
+                        spin_unlock(&sbi->fs_lock);
+                        /* Follow down to our covering mount. */
+                        if (!follow_down(&nd->path.mnt, &nd->path.dentry))
+                                goto done;
+                        /*
+                         * We shouldn't need to do this but we have no way
+                         * of knowing what may have been done so try a follow
+                         * just in case.
+                         */
+                        autofs4_follow_mount(&nd->path.mnt, &nd->path.dentry);
+                        goto done;
+                }
+                spin_unlock(&sbi->fs_lock);
                 goto done;
+        }
 
-        /* If an expire request is pending wait for it. */
+        /* If an expire request is pending everyone must wait. */
         spin_lock(&sbi->fs_lock);
         if (ino->flags & AUTOFS_INF_EXPIRING) {
                 spin_unlock(&sbi->fs_lock);
@@ -243,6 +262,7 @@ static void *autofs4_follow_link(struct dentry *dentry, struct nameidata *nd)
                         dentry, dentry->d_name.len, dentry->d_name.name);
 
                 status = autofs4_wait(sbi, dentry, NFY_NONE);
+                wait_for_completion(&ino->expire_complete);
 
                 DPRINTK("request done status=%d", status);
 
@@ -250,10 +270,15 @@ static void *autofs4_follow_link(struct dentry *dentry, struct nameidata *nd)
         }
         spin_unlock(&sbi->fs_lock);
 cont:
+        /* We trigger a mount for almost all flags */
+        lookup_type = nd->flags & (TRIGGER_FLAGS | TRIGGER_INTENTS);
+        if (!(lookup_type || dentry->d_flags & DCACHE_AUTOFS_PENDING))
+                goto done;
+
         /*
-         * If the dentry contains directories then it is an
-         * autofs multi-mount with no root mount offset. So
-         * don't try to mount it again.
+         * If the dentry contains directories then it is an autofs
+         * multi-mount with no root mount offset. So don't try to
+         * mount it again.
          */
         spin_lock(&dcache_lock);
         if (dentry->d_flags & DCACHE_AUTOFS_PENDING ||
@@ -264,22 +289,22 @@ cont:
                 if (status)
                         goto out_error;
 
-                /*
-                 * The mount succeeded but if there is no root mount
-                 * it must be an autofs multi-mount with no root offset
-                 * so we don't need to follow the mount.
-                 */
-                if (d_mountpoint(dentry)) {
-                        if (!autofs4_follow_mount(&nd->path.mnt,
-                                                  &nd->path.dentry)) {
-                                status = -ENOENT;
-                                goto out_error;
-                        }
-                }
-
-                goto done;
+                goto follow;
         }
         spin_unlock(&dcache_lock);
+follow:
+        /*
+         * If there is no root mount it must be an autofs
+         * multi-mount with no root offset so we don't need
+         * to follow it.
+         */
+        if (d_mountpoint(dentry)) {
+                if (!autofs4_follow_mount(&nd->path.mnt,
+                                          &nd->path.dentry)) {
+                        status = -ENOENT;
+                        goto out_error;
+                }
+        }
 
 done:
         return NULL;
@@ -545,6 +570,7 @@ static struct dentry *autofs4_lookup(struct inode *dir, struct dentry *dentry, s
                                 expiring, expiring->d_name.len,
                                 expiring->d_name.name);
                         autofs4_wait(sbi, expiring, NFY_NONE);
+                        wait_for_completion(&ino->expire_complete);
                         DPRINTK("request completed");
                         goto cont;
                 }