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A complicated deadlock exists when using the journal and an elevated
group_thrtead_cnt. It was found with loop devices, but its not clear
whether it can be seen with real disks. The deadlock can occur simply
by writing data with an fio script.
When the deadlock occurs, multiple threads will hang in different ways:
1) The group threads will hang in the blk-wbt code with bios waiting to
be submitted to the block layer:
io_schedule+0x70/0xb0
rq_qos_wait+0x153/0x210
wbt_wait+0x115/0x1b0
io_schedule+0x70/0xb0
rq_qos_wait+0x153/0x210
wbt_wait+0x115/0x1b0
__rq_qos_throttle+0x38/0x60
blk_mq_submit_bio+0x589/0xcd0
wbt_wait+0x115/0x1b0
__rq_qos_throttle+0x38/0x60
blk_mq_submit_bio+0x589/0xcd0
__submit_bio+0xe6/0x100
submit_bio_noacct_nocheck+0x42e/0x470
submit_bio_noacct+0x4c2/0xbb0
ops_run_io+0x46b/0x1a30
handle_stripe+0xcd3/0x36b0
handle_active_stripes.constprop.0+0x6f6/0xa60
raid5_do_work+0x177/0x330
Or:
io_schedule+0x70/0xb0
rq_qos_wait+0x153/0x210
wbt_wait+0x115/0x1b0
__rq_qos_throttle+0x38/0x60
blk_mq_submit_bio+0x589/0xcd0
__submit_bio+0xe6/0x100
submit_bio_noacct_nocheck+0x42e/0x470
submit_bio_noacct+0x4c2/0xbb0
flush_deferred_bios+0x136/0x170
raid5_do_work+0x262/0x330
2) The r5l_reclaim thread will hang in the same way, submitting a
bio to the block layer:
io_schedule+0x70/0xb0
rq_qos_wait+0x153/0x210
wbt_wait+0x115/0x1b0
__rq_qos_throttle+0x38/0x60
blk_mq_submit_bio+0x589/0xcd0
__submit_bio+0xe6/0x100
submit_bio_noacct_nocheck+0x42e/0x470
submit_bio_noacct+0x4c2/0xbb0
submit_bio+0x3f/0xf0
md_super_write+0x12f/0x1b0
md_update_sb.part.0+0x7c6/0xff0
md_update_sb+0x30/0x60
r5l_do_reclaim+0x4f9/0x5e0
r5l_reclaim_thread+0x69/0x30b
However, before hanging, the MD_SB_CHANGE_PENDING flag will be
set for sb_flags in r5l_write_super_and_discard_space(). This
flag will never be cleared because the submit_bio() call never
returns.
3) Due to the MD_SB_CHANGE_PENDING flag being set, handle_stripe()
will do no processing on any pending stripes and re-set
STRIPE_HANDLE. This will cause the raid5d thread to enter an
infinite loop, constantly trying to handle the same stripes
stuck in the queue.
The raid5d thread has a blk_plug that holds a number of bios
that are also stuck waiting seeing the thread is in a loop
that never schedules. These bios have been accounted for by
blk-wbt thus preventing the other threads above from
continuing when they try to submit bios. --Deadlock.
To fix this, add the same wait_event() that is used in raid5_do_work()
to raid5d() such that if MD_SB_CHANGE_PENDING is set, the thread will
schedule and wait until the flag is cleared. The schedule action will
flush the plug which will allow the r5l_reclaim thread to continue,
thus preventing the deadlock.
However, md_check_recovery() calls can also clear MD_SB_CHANGE_PENDING
from the same thread and can thus deadlock if the thread is put to
sleep. So avoid waiting if md_check_recovery() is being called in the
loop.
It's not clear when the deadlock was introduced, but the similar
wait_event() call in raid5_do_work() was added in 2017 by this
commit:
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|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.