[RFCv2 1/7] lockd: fix race in async lock request handling

[Alexander Aring <aahringo@redhat.com>]

This patch fixes a race in async lock request handling between adding
the relevant struct nlm_block to nlm_blocked list after the request was
sent by vfs_lock_file() and nlmsvc_grant_deferred() does a lookup of the
nlm_block in the nlm_blocked list. It could be that the async request is
completed before the nlm_block was added to the list. This would end
in a -ENOENT and a kernel log message of "lockd: grant for unknown
block".

To solve this issue we add the nlm_block before the vfs_lock_file() call
to be sure it has been added when a possible nlmsvc_grant_deferred() is
called. If the vfs_lock_file() results in an case when it wouldn't be
added to nlm_blocked list, the nlm_block struct will be removed from
this list again.

The introducing of the new B_PENDING_CALLBACK nlm_block flag will handle
async lock requests on a pending lock requests as a retry on the caller
level to hit the -EAGAIN case.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
---
 fs/lockd/svclock.c          | 100 ++++++++++++++++++++++++++----------
 include/linux/lockd/lockd.h |   2 +
 2 files changed, 74 insertions(+), 28 deletions(-)

diff --git a/fs/lockd/svclock.c b/fs/lockd/svclock.c
index c43ccdf28ed9..7d63524bdb81 100644
--- a/fs/lockd/svclock.c
+++ b/fs/lockd/svclock.c
@@ -133,6 +133,7 @@ nlmsvc_remove_block(struct nlm_block *block)
 {
 	if (!list_empty(&block->b_list)) {
 		spin_lock(&nlm_blocked_lock);
+		block->b_flags &= ~B_PENDING_CALLBACK;
 		list_del_init(&block->b_list);
 		spin_unlock(&nlm_blocked_lock);
 		nlmsvc_release_block(block);
@@ -232,6 +233,7 @@ nlmsvc_create_block(struct svc_rqst *rqstp, struct nlm_host *host,
 	kref_init(&block->b_count);
 	INIT_LIST_HEAD(&block->b_list);
 	INIT_LIST_HEAD(&block->b_flist);
+	mutex_init(&block->b_cb_mutex);
 
 	if (!nlmsvc_setgrantargs(call, lock))
 		goto failed_free;
@@ -289,6 +291,8 @@ static void nlmsvc_free_block(struct kref *kref)
 
 	dprintk("lockd: freeing block %p...\n", block);
 
+	WARN_ON_ONCE(block->b_flags & B_PENDING_CALLBACK);
+
 	/* Remove block from file's list of blocks */
 	list_del_init(&block->b_flist);
 	mutex_unlock(&file->f_mutex);
@@ -532,6 +536,13 @@ nlmsvc_lock(struct svc_rqst *rqstp, struct nlm_file *file,
 		goto out;
 	}
 
+	mutex_lock(&block->b_cb_mutex);
+	if (block->b_flags & B_PENDING_CALLBACK)
+		goto pending_request;
+
+	/* Append to list of blocked */
+	nlmsvc_insert_block(block, NLM_NEVER);
+
 	if (!wait)
 		lock->fl.fl_flags &= ~FL_SLEEP;
 	mode = lock_to_openmode(&lock->fl);
@@ -541,9 +552,13 @@ nlmsvc_lock(struct svc_rqst *rqstp, struct nlm_file *file,
 	dprintk("lockd: vfs_lock_file returned %d\n", error);
 	switch (error) {
 		case 0:
+			nlmsvc_remove_block(block);
 			ret = nlm_granted;
-			goto out;
+			goto out_cb_mutex;
 		case -EAGAIN:
+			if (!wait)
+				nlmsvc_remove_block(block);
+pending_request:
 			/*
 			 * If this is a blocking request for an
 			 * already pending lock request then we need
@@ -552,26 +567,29 @@ nlmsvc_lock(struct svc_rqst *rqstp, struct nlm_file *file,
 			if (wait)
 				break;
 			ret = async_block ? nlm_lck_blocked : nlm_lck_denied;
-			goto out;
+			goto out_cb_mutex;
 		case FILE_LOCK_DEFERRED:
+			block->b_flags |= B_PENDING_CALLBACK;
+
 			if (wait)
 				break;
 			/* Filesystem lock operation is in progress
 			   Add it to the queue waiting for callback */
 			ret = nlmsvc_defer_lock_rqst(rqstp, block);
-			goto out;
+			goto out_cb_mutex;
 		case -EDEADLK:
+			nlmsvc_remove_block(block);
 			ret = nlm_deadlock;
-			goto out;
+			goto out_cb_mutex;
 		default:			/* includes ENOLCK */
+			nlmsvc_remove_block(block);
 			ret = nlm_lck_denied_nolocks;
-			goto out;
+			goto out_cb_mutex;
 	}
 
 	ret = nlm_lck_blocked;
-
-	/* Append to list of blocked */
-	nlmsvc_insert_block(block, NLM_NEVER);
+out_cb_mutex:
+	mutex_unlock(&block->b_cb_mutex);
 out:
 	mutex_unlock(&file->f_mutex);
 	nlmsvc_release_block(block);
@@ -728,34 +746,60 @@ nlmsvc_update_deferred_block(struct nlm_block *block, int result)
 		block->b_flags |= B_TIMED_OUT;
 }
 
+static int __nlmsvc_grant_deferred(struct nlm_block *block,
+				   struct file_lock *fl,
+				   int result)
+{
+	int rc = 0;
+
+	dprintk("lockd: nlmsvc_notify_blocked block %p flags %d\n",
+					block, block->b_flags);
+	if (block->b_flags & B_QUEUED) {
+		if (block->b_flags & B_TIMED_OUT) {
+			rc = -ENOLCK;
+			goto out;
+		}
+		nlmsvc_update_deferred_block(block, result);
+	} else if (result == 0)
+		block->b_granted = 1;
+
+	nlmsvc_insert_block_locked(block, 0);
+	svc_wake_up(block->b_daemon);
+out:
+	return rc;
+}
+
 static int nlmsvc_grant_deferred(struct file_lock *fl, int result)
 {
-	struct nlm_block *block;
-	int rc = -ENOENT;
+	struct nlm_block *iter, *block = NULL;
+	int rc;
 
 	spin_lock(&nlm_blocked_lock);
-	list_for_each_entry(block, &nlm_blocked, b_list) {
-		if (nlm_compare_locks(&block->b_call->a_args.lock.fl, fl)) {
-			dprintk("lockd: nlmsvc_notify_blocked block %p flags %d\n",
-							block, block->b_flags);
-			if (block->b_flags & B_QUEUED) {
-				if (block->b_flags & B_TIMED_OUT) {
-					rc = -ENOLCK;
-					break;
-				}
-				nlmsvc_update_deferred_block(block, result);
-			} else if (result == 0)
-				block->b_granted = 1;
-
-			nlmsvc_insert_block_locked(block, 0);
-			svc_wake_up(block->b_daemon);
-			rc = 0;
+	list_for_each_entry(iter, &nlm_blocked, b_list) {
+		if (nlm_compare_locks(&iter->b_call->a_args.lock.fl, fl)) {
+			kref_get(&iter->b_count);
+			block = iter;
 			break;
 		}
 	}
 	spin_unlock(&nlm_blocked_lock);
-	if (rc == -ENOENT)
-		printk(KERN_WARNING "lockd: grant for unknown block\n");
+
+	if (!block) {
+		pr_warn("lockd: grant for unknown pending block\n");
+		return -ENOENT;
+	}
+
+	/* don't interfere with nlmsvc_lock() */
+	mutex_lock(&block->b_cb_mutex);
+	block->b_flags &= ~B_PENDING_CALLBACK;
+
+	spin_lock(&nlm_blocked_lock);
+	WARN_ON_ONCE(list_empty(&block->b_list));
+	rc = __nlmsvc_grant_deferred(block, fl, result);
+	spin_unlock(&nlm_blocked_lock);
+	mutex_unlock(&block->b_cb_mutex);
+
+	nlmsvc_release_block(block);
 	return rc;
 }
 
diff --git a/include/linux/lockd/lockd.h b/include/linux/lockd/lockd.h
index f42594a9efe0..91f55458f5fc 100644
--- a/include/linux/lockd/lockd.h
+++ b/include/linux/lockd/lockd.h
@@ -185,10 +185,12 @@ struct nlm_block {
 	struct nlm_file *	b_file;		/* file in question */
 	struct cache_req *	b_cache_req;	/* deferred request handling */
 	struct cache_deferred_req * b_deferred_req;
+	struct mutex		b_cb_mutex;	/* callback mutex */
 	unsigned int		b_flags;	/* block flags */
 #define B_QUEUED		1	/* lock queued */
 #define B_GOT_CALLBACK		2	/* got lock or conflicting lock */
 #define B_TIMED_OUT		4	/* filesystem too slow to respond */
+#define B_PENDING_CALLBACK	8	/* pending callback for lock request */
 };
 
 /*
-- 
2.31.1