gfs2: Asynchronous withdraw

So far, withdraws are carried out in the context of the calling task.
When another task tries to withdraw while a withdraw is already
underway, that task blocks as well.  Change that to carry out withdraws
asynchronously in workqueue context and don't block the task triggering
the withdraw anymore.

Fixes: syzbot+6b156e132970e550194c@syzkaller.appspotmail.com
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

Author: Andreas Gruenbacher

fs/gfs2/incore.h

@@ -716,6 +716,7 @@ struct gfs2_sbd {
 	struct gfs2_glock *sd_rename_gl;
 	struct gfs2_glock *sd_freeze_gl;
 	struct work_struct sd_freeze_work;
+	struct work_struct sd_withdraw_work;
 	wait_queue_head_t sd_kill_wait;
 	wait_queue_head_t sd_async_glock_wait;
 	atomic_t sd_glock_disposal;

fs/gfs2/ops_fstype.c

@@ -1215,6 +1215,8 @@ static int gfs2_fill_super(struct super_block *sb, struct fs_context *fc)
 	if (error)
 		goto fail_debug;
 
+	INIT_WORK(&sdp->sd_withdraw_work, gfs2_withdraw_func);
+
 	error = init_locking(sdp, &mount_gh, DO);
 	if (error)
 		goto fail_lm;

fs/gfs2/super.c

@@ -603,7 +603,7 @@ static void gfs2_put_super(struct super_block *sb)
 		gfs2_quota_cleanup(sdp);
 	}
 
-	WARN_ON(gfs2_withdrawing(sdp));
+	flush_work(&sdp->sd_withdraw_work);
 
 	/*  At this point, we're through modifying the disk  */
 

fs/gfs2/util.c

@@ -309,43 +309,50 @@ void gfs2_lm(struct gfs2_sbd *sdp, const char *fmt, ...)
 	va_end(args);
 }
 
-void gfs2_withdraw(struct gfs2_sbd *sdp)
+void gfs2_withdraw_func(struct work_struct *work)
 {
+	struct gfs2_sbd *sdp = container_of(work, struct gfs2_sbd, sd_withdraw_work);
 	struct lm_lockstruct *ls = &sdp->sd_lockstruct;
 	const struct lm_lockops *lm = ls->ls_ops;
 
+	if (test_bit(SDF_KILL, &sdp->sd_flags))
+		return;
+
+	BUG_ON(sdp->sd_args.ar_debug);
+
+	signal_our_withdraw(sdp);
+
+	kobject_uevent(&sdp->sd_kobj, KOBJ_OFFLINE);
+
+	if (!strcmp(sdp->sd_lockstruct.ls_ops->lm_proto_name, "lock_dlm"))
+		wait_for_completion(&sdp->sd_wdack);
+
+	if (lm->lm_unmount)
+		lm->lm_unmount(sdp, false);
+	fs_err(sdp, "file system withdrawn\n");
+	clear_bit(SDF_WITHDRAW_IN_PROG, &sdp->sd_flags);
+}
+
+void gfs2_withdraw(struct gfs2_sbd *sdp)
+{
 	if (sdp->sd_args.ar_errors == GFS2_ERRORS_WITHDRAW) {
 		unsigned long old = READ_ONCE(sdp->sd_flags), new;
 
 		do {
-			if (old & BIT(SDF_WITHDRAWN)) {
-				wait_on_bit(&sdp->sd_flags,
-					    SDF_WITHDRAW_IN_PROG,
-					    TASK_UNINTERRUPTIBLE);
+			if (old & BIT(SDF_WITHDRAWN))
 				return;
-			}
 			new = old | BIT(SDF_WITHDRAWN) | BIT(SDF_WITHDRAW_IN_PROG);
 		} while (unlikely(!try_cmpxchg(&sdp->sd_flags, &old, new)));
 
-		fs_err(sdp, "about to withdraw this file system\n");
-		BUG_ON(sdp->sd_args.ar_debug);
-
-		signal_our_withdraw(sdp);
-
-		kobject_uevent(&sdp->sd_kobj, KOBJ_OFFLINE);
-
-		if (!strcmp(sdp->sd_lockstruct.ls_ops->lm_proto_name, "lock_dlm"))
-			wait_for_completion(&sdp->sd_wdack);
-
-		if (lm->lm_unmount) {
-			fs_err(sdp, "telling LM to unmount\n");
-			lm->lm_unmount(sdp, false);
-		}
-		fs_err(sdp, "File system withdrawn\n");
 		dump_stack();
-		clear_bit(SDF_WITHDRAW_IN_PROG, &sdp->sd_flags);
-		smp_mb__after_atomic();
-		wake_up_bit(&sdp->sd_flags, SDF_WITHDRAW_IN_PROG);
+		/*
+		 * There is no need to withdraw when the superblock hasn't been
+		 * fully initialized, yet.
+		 */
+		if (!(sdp->sd_vfs->s_flags & SB_BORN))
+			return;
+		fs_err(sdp, "about to withdraw this file system\n");
+		schedule_work(&sdp->sd_withdraw_work);
 	}
 
 	if (sdp->sd_args.ar_errors == GFS2_ERRORS_PANIC)

fs/gfs2/util.h

@@ -232,6 +232,8 @@ gfs2_tune_get_i(&(sdp)->sd_tune, &(sdp)->sd_tune.field)
 
 __printf(2, 3)
 void gfs2_lm(struct gfs2_sbd *sdp, const char *fmt, ...);
+
+void gfs2_withdraw_func(struct work_struct *work);
 void gfs2_withdraw(struct gfs2_sbd *sdp);
 
 #endif /* __UTIL_DOT_H__ */