autofs - use dentry flags to block walks during expire

Somewhere along the way the autofs expire operation has changed to hold a spin lock over expired dentry selection. The autofs indirect mount expired dentry selection is complicated and quite lengthy so it isn't appropriate to hold a spin lock over the operation.

Commit 47be6184 added a might_sleep() to dput() causing a BUG() about this usage to be issued.

But the spin lock doesn't need to be held over this check, the autofs dentry info. flags are enough to block walks into dentrys during the expire.

I've left the direct mount expire as it is (for now) becuase it is much simpler and quicker than the indirect mount expire and adding spin lock release and re-aquires would do nothing more than add overhead.

Fixes 47be61845c77 ("fs/dcache.c: avoid soft-lockup in dput()")

Signed-off-by Ian Kent <[email protected]>

Cc Takashi Iwai <[email protected]>

Cc Andrew Morton <[email protected]>

Cc NeilBrown <[email protected]>

---
 fs/autofs4/expire.c |   55 +++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 42 insertions(+), 13 deletions(-)

fs/autofs4/expire.c +42 -13

index b493909..d8e6d42 100644
@@ -417,6 +417,7 @@ static struct dentry *should_expire(struct dentry *dentry,
 	}
 	return NULL;
 }
+
 /*
  * Find an eligible tree to time-out
  * A tree is eligible if :-

@@ -432,6 +433,7 @@ struct dentry *autofs4_expire_indirect(struct super_block *sb,
 	struct dentry *root = sb->s_root;
 	struct dentry *dentry;
 	struct dentry *expired;
+	struct dentry *found;
 	struct autofs_info *ino;
 
 	if (!root)

@@ -442,31 +444,46 @@ struct dentry *autofs4_expire_indirect(struct super_block *sb,
 
 	dentry = NULL;
 	while ((dentry = get_next_positive_subdir(dentry, root))) {
+		int flags = how;
+
 		spin_lock(&sbi->fs_lock);
 		ino = autofs4_dentry_ino(dentry);
-		if (ino->flags & AUTOFS_INF_WANT_EXPIRE)
-			expired = NULL;
-		else
-			expired = should_expire(dentry, mnt, timeout, how);
-		if (!expired) {
+		if (ino->flags & AUTOFS_INF_WANT_EXPIRE) {
 			spin_unlock(&sbi->fs_lock);
 			continue;
 		}
+		spin_unlock(&sbi->fs_lock);
+
+		expired = should_expire(dentry, mnt, timeout, flags);
+		if (!expired)
+			continue;
+
+		spin_lock(&sbi->fs_lock);
 		ino = autofs4_dentry_ino(expired);
 		ino->flags |= AUTOFS_INF_WANT_EXPIRE;
 		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;
-		}
 
+		/* Make sure a reference is not taken on found if
+		 * things have changed.
+		 */
+		flags &= ~AUTOFS_EXP_LEAVES;
+		found = should_expire(expired, mnt, timeout, how);
+		if (!found || found != expired)
+			/* Something has changed, continue */
+			goto next;
+
+		if (expired != dentry)
+			dput(dentry);
+
+		spin_lock(&sbi->fs_lock);
+		goto found;
+next:
+		spin_lock(&sbi->fs_lock);
 		ino->flags &= ~AUTOFS_INF_WANT_EXPIRE;
+		spin_unlock(&sbi->fs_lock);
 		if (expired != dentry)
 			dput(expired);
-		spin_unlock(&sbi->fs_lock);
 	}
 	return NULL;
 

@@ -483,6 +500,7 @@ 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);
 	int status;
+	int state;
 
 	/* Block on any pending expire */
 	if (!(ino->flags & AUTOFS_INF_WANT_EXPIRE))

@@ -490,8 +508,19 @@ int autofs4_expire_wait(struct dentry *dentry, int rcu_walk)
 	if (rcu_walk)
 		return -ECHILD;
 
+retry:
 	spin_lock(&sbi->fs_lock);
-	if (ino->flags & AUTOFS_INF_EXPIRING) {
+	state = ino->flags & (AUTOFS_INF_WANT_EXPIRE | AUTOFS_INF_EXPIRING);
+	if (state == AUTOFS_INF_WANT_EXPIRE) {
+		spin_unlock(&sbi->fs_lock);
+		/*
+		 * Possibly being selected for expire, wait until
+		 * it's selected or not.
+		 */
+		schedule_timeout_uninterruptible(HZ/10);
+		goto retry;
+	}
+	if (state & AUTOFS_INF_EXPIRING) {
 		spin_unlock(&sbi->fs_lock);
 
 		pr_debug("waiting for expire %p name=%pd\n", dentry, dentry);

Somewhere along the way the autofs expire operation has changed to hold a spin lock over expired dentry selection. The autofs indirect mount expired dentry selection is complicated and quite lengthy so it isn't appropriate to hold a spin lock over the operation.

Commit 47be6184 added a might_sleep() to dput() causing a BUG() about this usage to be issued.

But the spin lock doesn't need to be held over this check, the autofs dentry info. flags are enough to block walks into dentrys during the expire.

I've left the direct mount expire as it is (for now) becuase it is much simpler and quicker than the indirect mount expire and adding spin lock release and re-aquires would do nothing more than add overhead.

Fixes 47be61845c77 ("fs/dcache.c: avoid soft-lockup in dput()")

Signed-off-by Ian Kent <[email protected]>

Cc Takashi Iwai <[email protected]>

Cc Andrew Morton <[email protected]>

Cc NeilBrown <[email protected]>