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@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx> --- 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
