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From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
To: Jan Kara <jack@suse.cz>
Cc: syzbot <syzbot+4a7438e774b21ddd8eca@syzkaller.appspotmail.com>,
	syzkaller-bugs@googlegroups.com, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, viro@zeniv.linux.org.uk,
	axboe@kernel.dk, tj@kernel.org, david@fromorbit.com,
	linux-block@vger.kernel.org
Subject: Re: general protection fault in wb_workfn (2)
Date: Thu, 31 May 2018 01:00:08 +0900	[thread overview]
Message-ID: <2dda7a11-3f6b-bdba-a68a-7c0694806cc4@I-love.SAKURA.ne.jp> (raw)
In-Reply-To: <20180528133503.awomzj6djozbo5bv@quack2.suse.cz>

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

  reply	other threads:[~2018-05-30 16:01 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-26  9:15 general protection fault in wb_workfn (2) syzbot
2018-05-27  0:47 ` Tetsuo Handa
2018-05-27  2:21   ` [PATCH] bdi: Fix another oops in wb_workfn() Tetsuo Handa
2018-05-27  2:36     ` Tejun Heo
2018-05-28 13:35   ` general protection fault in wb_workfn (2) Jan Kara
2018-05-30 16:00     ` Tetsuo Handa [this message]
2018-05-31 11:42       ` Jan Kara
2018-05-31 13:19         ` Tetsuo Handa
2018-05-31 13:42           ` Jan Kara
2018-05-31 16:56             ` Jens Axboe
2018-06-05 13:45               ` Tetsuo Handa
2018-06-07 18:46                 ` Dmitry Vyukov
2018-06-08  2:31                   ` Tetsuo Handa
2018-06-08 14:45                     ` Dmitry Vyukov
2018-06-08 15:16                       ` Dmitry Vyukov
2018-06-08 16:53                         ` Dmitry Vyukov
2018-06-08 17:14                           ` Dmitry Vyukov
2018-06-09  5:30                             ` Tetsuo Handa
2018-06-09 14:00                               ` [PATCH] bdi: Fix another oops in wb_workfn() Tetsuo Handa
2018-06-11  9:12                                 ` Jan Kara
2018-06-11 16:01                                   ` Tejun Heo
2018-06-11 16:29                                     ` Jan Kara
2018-06-11 17:20                                       ` Tejun Heo
2018-06-12 15:57                                         ` Jan Kara
2018-06-13 10:43                                           ` Tetsuo Handa
2018-06-13 11:51                                             ` Tetsuo Handa
2018-06-13 14:06                                             ` Linus Torvalds
2018-06-13 14:46                                             ` Jan Kara
2018-06-13 14:55                                               ` Linus Torvalds
2018-06-13 16:20                                               ` Tetsuo Handa
2018-06-13 16:25                                                 ` Linus Torvalds
2018-06-13 16:45                                                   ` Jan Kara
2018-06-13 21:04                                                     ` Tetsuo Handa
2018-06-14 10:11                                                       ` Jan Kara
2018-06-13 14:33                                           ` Tejun Heo
2018-06-15 12:06                                             ` Jan Kara
2018-06-18 12:27                                               ` Jan Kara
2018-06-01  2:30             ` general protection fault in wb_workfn (2) Dave Chinner

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