[PATCH v2] bdi: Add temporary config for debugging wb_workfn() versus bdi_unregister() race bug.

[Tetsuo Handa]

syzbot is hitting NULL pointer dereference at wb_workfn() [1]. But due to
limitations that syzbot cannot find reproducer for this bug (frequency is
once or twice per a day) nor we can't capture vmcore in the environment
which syzbot is using, for now we need to rely on printk() debugging.

[1] https://syzkaller.appspot.com/bug?id=e0818ccb7e46190b3f1038b0c794299208ed4206

Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
---
 block/Kconfig     |  7 +++++++
 fs/fs-writeback.c | 28 ++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/block/Kconfig b/block/Kconfig
index 28ec557..fbce13e 100644
--- a/block/Kconfig
+++ b/block/Kconfig
@@ -139,6 +139,13 @@ config BLK_CMDLINE_PARSER
 
 	See Documentation/block/cmdline-partition.txt for more information.
 
+config BLK_DEBUG_WB_WORKFN_RACE
+	bool "Dump upon hitting wb_workfn() versus bdi_unregister() race bug."
+	default n
+	---help---
+	This is a temporary option used for obtaining information for
+	specific bug. This option will be removed after the bug is fixed.
+
 config BLK_WBT
 	bool "Enable support for block device writeback throttling"
 	default n
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 471d863..14ab873 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1934,6 +1934,34 @@ void wb_workfn(struct work_struct *work)
 						struct bdi_writeback, dwork);
 	long pages_written;
 
+#ifdef CONFIG_BLK_DEBUG_WB_WORKFN_RACE
+	if (!wb->bdi->dev) {
+		pr_warn("WARNING: %s: device is NULL\n", __func__);
+		pr_warn("wb->state=%lx\n", wb->state);
+		pr_warn("(wb == &wb->bdi->wb)=%u\n", wb == &wb->bdi->wb);
+		pr_warn("list_empty(&wb->work_list)=%u\n",
+			list_empty(&wb->work_list));
+		pr_warn("list_empty(&wb->bdi->bdi_list)=%u\n",
+			list_empty(&wb->bdi->bdi_list));
+		pr_warn("wb->bdi->wb.state=%lx\n", wb->bdi->wb.state);
+		if (!wb->congested)
+			pr_warn("wb->congested == NULL\n");
+#ifdef CONFIG_CGROUP_WRITEBACK
+		else if (!wb->congested->__bdi)
+			pr_warn("wb->congested->__bdi == NULL\n");
+		else {
+			pr_warn("(wb->congested->__bdi == wb->bdi)=%u\n",
+				wb->congested->__bdi == wb->bdi);
+			pr_warn("list_empty(&wb->congested->__bdi->bdi_list)=%u\n",
+				list_empty(&wb->congested->__bdi->bdi_list));
+			pr_warn("wb->congested->__bdi->wb.state=%lx\n",
+				wb->congested->__bdi->wb.state);
+		}
+#endif
+		/* Will halt shortly due to NULL pointer dereference... */
+	}
+#endif
+
 	set_worker_desc("flush-%s", dev_name(wb->bdi->dev));
 	current->flags |= PF_SWAPWRITE;
 
-- 
1.8.3.1