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From:   Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Subject: Re: general protection fault in wb_workfn (2)
To:     Jan Kara <jack@suse.cz>
Cc:     syzbot <syzbot+4a7438e774b21ddd8eca@syzkaller.appspotmail.com>,
        syzkaller-bugs@googlegroups.com, linux-fsdevel@vger.kernel.org,
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        axboe@kernel.dk, tj@kernel.org, david@fromorbit.com,
        linux-block@vger.kernel.org
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 <0c7c5dea-7312-8a59-9d1b-5467f69719bf@I-love.SAKURA.ne.jp>
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Date:   Thu, 31 May 2018 01:00:08 +0900
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So, we have no idea what is happening...
Then, what about starting from temporary debug printk() patch shown below?

>From 4f70f72ad3c9ae6ce1678024ef740aca4958e5b0 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Wed, 30 May 2018 09:57:10 +0900
Subject: [PATCH] bdi: Add temporary config for debugging wb_workfn() versus
 bdi_unregister() race bug.

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@I-love.SAKURA.ne.jp>
---
 block/Kconfig     |  7 +++++++
 fs/fs-writeback.c | 31 +++++++++++++++++++++++++++++++
 2 files changed, 38 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..b4dd078 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1934,6 +1934,37 @@ 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("list_empty(&wb->work_list)=%u\n",
+			list_empty(&wb->work_list));
+		if (!wb->bdi)
+			pr_warn("wb->bdi == NULL\n");
+		else {
+			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