bcache: Convert writeback to a kthread

This simplifies the writeback flow control quite a bit - previously, it
was conceptually two coroutines, refill_dirty() and read_dirty(). This
makes the code quite a bit more straightforward.
Signed-off-by: Kent Overstreet <kmo@daterainc.com>

drivers/md/bcache/bcache.h

@@ -509,7 +509,7 @@ struct cached_dev {
 
 	/* Limit number of writeback bios in flight */
 	struct semaphore	in_flight;
-	struct closure_with_timer writeback;
+	struct task_struct	*writeback_thread;
 
 	struct keybuf		writeback_keys;
 
@@ -1038,7 +1038,11 @@ static inline void bkey_init(struct bkey *k)
 
 #define KEY_START(k)		(KEY_OFFSET(k) - KEY_SIZE(k))
 #define START_KEY(k)		KEY(KEY_INODE(k), KEY_START(k), 0)
-#define MAX_KEY			KEY(~(~0 << 20), ((uint64_t) ~0) >> 1, 0)
+
+#define MAX_KEY_INODE		(~(~0 << 20))
+#define MAX_KEY_OFFSET		(((uint64_t) ~0) >> 1)
+#define MAX_KEY			KEY(MAX_KEY_INODE, MAX_KEY_OFFSET, 0)
+
 #define ZERO_KEY		KEY(0, 0, 0)
 
 /*
@@ -1214,8 +1218,6 @@ int bch_cache_allocator_init(struct cache *ca);
 
 void bch_debug_exit(void);
 int bch_debug_init(struct kobject *);
-void bch_writeback_exit(void);
-int bch_writeback_init(void);
 void bch_request_exit(void);
 int bch_request_init(void);
 void bch_btree_exit(void);

drivers/md/bcache/super.c

@@ -1029,6 +1029,7 @@ static void cached_dev_free(struct closure *cl)
 	struct cached_dev *dc = container_of(cl, struct cached_dev, disk.cl);
 
 	cancel_delayed_work_sync(&dc->writeback_rate_update);
+	kthread_stop(dc->writeback_thread);
 
 	mutex_lock(&bch_register_lock);
 
@@ -2006,7 +2007,6 @@ static struct notifier_block reboot = {
 static void bcache_exit(void)
 {
 	bch_debug_exit();
-	bch_writeback_exit();
 	bch_request_exit();
 	bch_btree_exit();
 	if (bcache_kobj)
@@ -2039,7 +2039,6 @@ static int __init bcache_init(void)
 	    sysfs_create_files(bcache_kobj, files) ||
 	    bch_btree_init() ||
 	    bch_request_init() ||
-	    bch_writeback_init() ||
 	    bch_debug_init(bcache_kobj))
 		goto err;
 

drivers/md/bcache/writeback.h

@@ -56,11 +56,30 @@ static inline bool should_writeback(struct cached_dev *dc, struct bio *bio,
 		in_use <= CUTOFF_WRITEBACK;
 }
 
+static inline void bch_writeback_queue(struct cached_dev *dc)
+{
+	wake_up_process(dc->writeback_thread);
+}
+
+static inline void bch_writeback_add(struct cached_dev *dc)
+{
+	if (!atomic_read(&dc->has_dirty) &&
+	    !atomic_xchg(&dc->has_dirty, 1)) {
+		atomic_inc(&dc->count);
+
+		if (BDEV_STATE(&dc->sb) != BDEV_STATE_DIRTY) {
+			SET_BDEV_STATE(&dc->sb, BDEV_STATE_DIRTY);
+			/* XXX: should do this synchronously */
+			bch_write_bdev_super(dc, NULL);
+		}
+
+		bch_writeback_queue(dc);
+	}
+}
+
 void bcache_dev_sectors_dirty_add(struct cache_set *, unsigned, uint64_t, int);
-void bch_writeback_queue(struct cached_dev *);
-void bch_writeback_add(struct cached_dev *);
 
 void bch_sectors_dirty_init(struct cached_dev *dc);
-void bch_cached_dev_writeback_init(struct cached_dev *);
+int bch_cached_dev_writeback_init(struct cached_dev *);
 
 #endif