gfs2: use workqueue instead of slow-work

Workqueue can now handle high concurrency.  Convert gfs to use
workqueue instead of slow-work.

* Steven pointed out that recovery path might be run from allocation
  path and thus requires forward progress guarantee without memory
  allocation.  Create and use gfs_recovery_wq with rescuer.  Please
  note that forward progress wasn't guaranteed with slow-work.

* Updated to use non-reentrant workqueue.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Steven Whitehouse <swhiteho@redhat.com>
diff --git a/fs/gfs2/main.c b/fs/gfs2/main.c
index fb2a5f9..b1e9630 100644
--- a/fs/gfs2/main.c
+++ b/fs/gfs2/main.c
@@ -15,7 +15,6 @@
 #include <linux/init.h>
 #include <linux/gfs2_ondisk.h>
 #include <asm/atomic.h>
-#include <linux/slow-work.h>
 
 #include "gfs2.h"
 #include "incore.h"
@@ -24,6 +23,7 @@
 #include "util.h"
 #include "glock.h"
 #include "quota.h"
+#include "recovery.h"
 
 static struct shrinker qd_shrinker = {
 	.shrink = gfs2_shrink_qd_memory,
@@ -138,9 +138,11 @@
 	if (error)
 		goto fail_unregister;
 
-	error = slow_work_register_user(THIS_MODULE);
-	if (error)
-		goto fail_slow;
+	error = -ENOMEM;
+	gfs_recovery_wq = alloc_workqueue("gfs_recovery",
+					  WQ_NON_REENTRANT | WQ_RESCUER, 0);
+	if (!gfs_recovery_wq)
+		goto fail_wq;
 
 	gfs2_register_debugfs();
 
@@ -148,7 +150,7 @@
 
 	return 0;
 
-fail_slow:
+fail_wq:
 	unregister_filesystem(&gfs2meta_fs_type);
 fail_unregister:
 	unregister_filesystem(&gfs2_fs_type);
@@ -190,7 +192,7 @@
 	gfs2_unregister_debugfs();
 	unregister_filesystem(&gfs2_fs_type);
 	unregister_filesystem(&gfs2meta_fs_type);
-	slow_work_unregister_user(THIS_MODULE);
+	destroy_workqueue(gfs_recovery_wq);
 
 	kmem_cache_destroy(gfs2_quotad_cachep);
 	kmem_cache_destroy(gfs2_rgrpd_cachep);