fuse: fix hang of single threaded fuseblk filesystem

Single threaded NTFS-3G could get stuck if a delayed RELEASE reply
triggered a DESTROY request via path_put().

Fix this by

 a) making RELEASE requests synchronous, whenever possible, on fuseblk
 filesystems

 b) if not possible (triggered by an asynchronous read/write) then do
 the path_put() in a separate thread with schedule_work().
Reported-by: Oliver Neukum <oneukum@suse.de>
Cc: stable@kernel.org
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/fuse/file.c

@@ -86,18 +86,52 @@ struct fuse_file *fuse_file_get(struct fuse_file *ff)
 	return ff;
 }
 
+static void fuse_release_async(struct work_struct *work)
+{
+	struct fuse_req *req;
+	struct fuse_conn *fc;
+	struct path path;
+
+	req = container_of(work, struct fuse_req, misc.release.work);
+	path = req->misc.release.path;
+	fc = get_fuse_conn(path.dentry->d_inode);
+
+	fuse_put_request(fc, req);
+	path_put(&path);
+}
+
 static void fuse_release_end(struct fuse_conn *fc, struct fuse_req *req)
 {
+	if (fc->destroy_req) {
+		/*
+		 * If this is a fuseblk mount, then it's possible that
+		 * releasing the path will result in releasing the
+		 * super block and sending the DESTROY request.  If
+		 * the server is single threaded, this would hang.
+		 * For this reason do the path_put() in a separate
+		 * thread.
+		 */
+		atomic_inc(&req->count);
+		INIT_WORK(&req->misc.release.work, fuse_release_async);
+		schedule_work(&req->misc.release.work);
+	} else {
 		path_put(&req->misc.release.path);
+	}
 }
 
-static void fuse_file_put(struct fuse_file *ff)
+static void fuse_file_put(struct fuse_file *ff, bool sync)
 {
 	if (atomic_dec_and_test(&ff->count)) {
 		struct fuse_req *req = ff->reserved_req;
 
+		if (sync) {
+			fuse_request_send(ff->fc, req);
+			path_put(&req->misc.release.path);
+			fuse_put_request(ff->fc, req);
+		} else {
 			req->end = fuse_release_end;
 			fuse_request_send_background(ff->fc, req);
+		}
 		kfree(ff);
 	}
 }
@@ -219,8 +253,12 @@ void fuse_release_common(struct file *file, int opcode)
 	 * Normally this will send the RELEASE request, however if
 	 * some asynchronous READ or WRITE requests are outstanding,
 	 * the sending will be delayed.
+	 *
+	 * Make the release synchronous if this is a fuseblk mount,
+	 * synchronous RELEASE is allowed (and desirable) in this case
+	 * because the server can be trusted not to screw up.
 	 */
-	fuse_file_put(ff);
+	fuse_file_put(ff, ff->fc->destroy_req != NULL);
 }
 
 static int fuse_open(struct inode *inode, struct file *file)
@@ -558,7 +596,7 @@ static void fuse_readpages_end(struct fuse_conn *fc, struct fuse_req *req)
 		page_cache_release(page);
 	}
 	if (req->ff)
-		fuse_file_put(req->ff);
+		fuse_file_put(req->ff, false);
 }
 
 static void fuse_send_readpages(struct fuse_req *req, struct file *file)
@@ -1137,7 +1175,7 @@ static ssize_t fuse_direct_write(struct file *file, const char __user *buf,
 static void fuse_writepage_free(struct fuse_conn *fc, struct fuse_req *req)
 {
 	__free_page(req->pages[0]);
-	fuse_file_put(req->ff);
+	fuse_file_put(req->ff, false);
 }
 
 static void fuse_writepage_finish(struct fuse_conn *fc, struct fuse_req *req)

fs/fuse/fuse_i.h

@@ -21,6 +21,7 @@
 #include <linux/rwsem.h>
 #include <linux/rbtree.h>
 #include <linux/poll.h>
+#include <linux/workqueue.h>
 
 /** Max number of pages that can be used in a single read request */
 #define FUSE_MAX_PAGES_PER_REQ 32
@@ -262,7 +263,10 @@ struct fuse_req {
 	/** Data for asynchronous requests */
 	union {
 		struct {
+			union {
 				struct fuse_release_in in;
+				struct work_struct work;
+			};
 			struct path path;
 		} release;
 		struct fuse_init_in init_in;