[[PATCH v3] sched/core: Always flush pending blk_plug] On 08/07/2022 (Fri 17:27) John Keeping wrote:
> With CONFIG_PREEMPT_RT, it is possible to hit a deadlock between two
> normal priority tasks (SCHED_OTHER, nice level zero):
>
> INFO: task kworker/u8:0:8 blocked for more than 491 seconds.
> Not tainted 5.15.49-rt46 #1
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> task:kworker/u8:0 state:D stack: 0 pid: 8 ppid: 2 flags:0x00000000
> Workqueue: writeback wb_workfn (flush-7:0)
This is great. I was on-and-off looking at an ext4 deadlock triggered
by LTP's "ftest01" - and was just potting my notes together this morning
to clarify for myself what I thought was going on before I forgot.
Which is good, because I then could recognize the very similar patterns
in what you saw to what I'd observed/deduced. Otherise I'd probably not
have had your backtraces grab my attention at all.
Long story short, I applied this to v5.19-rc3-rt5 and it resolves the
LTP/ext4 deadlock issue. So...
Tested-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
As I'd already put together my ext4 debug info, I'll include it below
in case it helps future people searching for LTP and ftest01.
Thanks,
Paul.
-----------------
There is a reproducible deadlock in ext4 on preempt-rt triggered by LTP
"ftest01"
Reproducer details are at the bottom. What seems to happen is as follows:
The kworker doing writeback wb_workfn (pid 213 below) stalls on the
one and only down_write in ext4_map_blocks() and never moves again.
/*
* New blocks allocate and/or writing to unwritten extent
* will possibly result in updating i_data, so we take
* the write lock of i_data_sem, and call get_block()
* with create == 1 flag.
*/
down_write(&EXT4_I(inode)->i_data_sem);
So who is blocking the kworker? There are two of the ftest01 processes
(pid 818 and pid 821) doing ext4 truncate which look like this:
ext4_truncate+0x46a/0x660
--> ext4_ext_truncate+0xd6/0xf0
--> ext4_ext_remove_space+0x59f/0x1830
--> jbd2_journal_get_write_access+0x51/0x80
--> do_get_write_access+0x2ad/0x550
--> complete_all+0x60/0x60
--> bit_wait_io+0x11/0x60
They too are stalled in D state waiting there forever - presumably
waiting on the kwowrker to make the progress they are waiting for.
Here is where I think lockdep shows us the problem:
5 locks held by ftest01/818:
[...]
#4: ffff957c2a679958 (&ei->i_data_sem){++++}-{3:3}, at: ext4_truncate+0x443/0x660
5 locks held by ftest01/821:
[...]
#4: ffff957c2a67bb78 (&ei->i_data_sem){++++}-{3:3}, at: ext4_truncate+0x443/0x660
Yes - that is the one the kworker was trying the down_write() on.
Also note that the bit_wait is a big telltale sign here - there is only
one in the jbd2's do_get_write_access() function:
/*
* There is one case we have to be very careful about. If the
* committing transaction is currently writing this buffer out to disk
* and has NOT made a copy-out, then we cannot modify the buffer
* contents at all right now. The essence of copy-out is that it is
* the extra copy, not the primary copy, which gets journaled. If the
* primary copy is already going to disk then we cannot do copy-out
* here.
*/
if (buffer_shadow(bh)) {
JBUFFER_TRACE(jh, "on shadow: sleep");
spin_unlock(&jh->b_state_lock);
wait_on_bit_io(&bh->b_state, BH_Shadow, TASK_UNINTERRUPTIBLE);
goto repeat;
}
Presumaby this is a corner case that preempt-rt makes easier to trigger?
The full trace with interspersed lockdep info follows.
Various detailed reproducer information follows that.
-----------------------
sysrq-w backtracce information interspersed with sysrq-d lockdep info
on 5.19-rc2-rt3 kernel:
task:kworker/u129:34 state:D stack: 0 pid: 213 ppid: 2 flags:0x00004000
Workqueue: writeback wb_workfn (flush-7:0)
Call Trace:
<TASK>
__schedule+0x329/0xae0
? __this_cpu_preempt_check+0x13/0x20
? __rt_mutex_slowlock.isra.21.constprop.25+0xc0e/0xc40
schedule+0x48/0xc0
__rt_mutex_slowlock.isra.21.constprop.25+0x1b8/0xc40
rwbase_write_lock+0x98/0x6e0
down_write+0x46/0x50
? ext4_map_blocks+0x156/0x5c0
ext4_map_blocks+0x156/0x5c0
ext4_writepages+0xa1a/0x1290
? __this_cpu_preempt_check+0x13/0x20
? lockdep_hardirqs_on+0xcd/0x150
do_writepages+0xd2/0x1a0
__writeback_single_inode+0x64/0x850
writeback_sb_inodes+0x22d/0x5b0
__writeback_inodes_wb+0x67/0xe0
wb_writeback+0x269/0x5f0
? get_nr_inodes+0x49/0x70
wb_workfn+0x43d/0x780
? __this_cpu_preempt_check+0x13/0x20
process_one_work+0x286/0x710
worker_thread+0x3c/0x3d0
? process_one_work+0x710/0x710
kthread+0x13b/0x150
? kthread_complete_and_exit+0x20/0x20
ret_from_fork+0x1f/0x30
</TASK>
6 locks held by kworker/u129:34/213:
#0: ffff957c25e9ef28 ((wq_completion)writeback){+.+.}-{0:0}, at: process_one_work+0x201/0x710
#1: ffffb7e9c3797e68 ((work_completion)(&(&wb->dwork)->work)){+.+.}-{0:0}, at: process_one_work+0x201/0x710
#2: ffff957b9c9b70d0 (&type->s_umount_key#34){++++}-{3:3}, at: trylock_super+0x1b/0x50
#3: ffff957b8a9b2d18 (&sbi->s_writepages_rwsem){.+.+}-{0:0}, at: do_writepages+0xd2/0x1a0
#4: ffff957b9c9b5be8 (jbd2_handle){++++}-{0:0}, at: start_this_handle+0x13e/0x5f0
#5: ffff957c2a679958 (&ei->i_data_sem){++++}-{3:3}, at: ext4_map_blocks+0x156/0x5c0
task:jbd2/loop0-8 state:D stack: 0 pid: 679 ppid: 2 flags:0x00004000
Call Trace:
<TASK>
__schedule+0x329/0xae0
schedule+0x48/0xc0
jbd2_journal_wait_updates+0x7c/0xf0
? schedstat_stop+0x10/0x10
jbd2_journal_commit_transaction+0x284/0x20e0
? __this_cpu_preempt_check+0x13/0x20
? find_held_lock+0x35/0xa0
? _raw_spin_unlock_irqrestore+0x86/0x8f
? _raw_spin_unlock_irqrestore+0x86/0x8f
? try_to_del_timer_sync+0x53/0x80
? trace_hardirqs_on+0x3b/0x120
? try_to_del_timer_sync+0x53/0x80
kjournald2+0xd5/0x2b0
? schedstat_stop+0x10/0x10
? commit_timeout+0x20/0x20
kthread+0x13b/0x150
? kthread_complete_and_exit+0x20/0x20
ret_from_fork+0x1f/0x30
</TASK>
task:ftest01 state:D stack: 0 pid: 817 ppid: 816 flags:0x00004002
Call Trace:
<TASK>
__schedule+0x329/0xae0
? __x64_sys_tee+0xd0/0xd0
? __x64_sys_tee+0xd0/0xd0
schedule+0x48/0xc0
wb_wait_for_completion+0x62/0x90
? schedstat_stop+0x10/0x10
sync_inodes_sb+0xdd/0x460
? __this_cpu_preempt_check+0x13/0x20
? __x64_sys_tee+0xd0/0xd0
sync_inodes_one_sb+0x15/0x20
iterate_supers+0x94/0x100
ksys_sync+0x42/0xa0
__do_sys_sync+0xe/0x20
do_syscall_64+0x3f/0x90
entry_SYSCALL_64_after_hwframe+0x46/0xb0
RIP: 0033:0x7f3f9dae2cdb
RSP: 002b:00007ffe06430318 EFLAGS: 00000246 ORIG_RAX: 00000000000000a2
RAX: ffffffffffffffda RBX: 0000000000000800 RCX: 00007f3f9dae2cdb
RDX: 0000000000000040 RSI: 0000561f57de52f0 RDI: 0000561f57de5340
RBP: 00000000000ff000 R08: 0000000000000000 R09: 00007f3f9dbce0a0
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000207 R14: 0000000000000049 R15: 0000000000000003
</TASK>
1 lock held by ftest01/817:
#0: ffff957b9c9b70d0 (&type->s_umount_key#34){++++}-{3:3}, at: iterate_supers+0x7d/0x100
task:ftest01 state:D stack: 0 pid: 818 ppid: 816 flags:0x00004002
Call Trace:
<TASK>
__schedule+0x329/0xae0
? bit_wait+0x60/0x60
schedule+0x48/0xc0
io_schedule+0x16/0x40
bit_wait_io+0x11/0x60
__wait_on_bit_lock+0x5e/0xb0
out_of_line_wait_on_bit_lock+0x93/0xb0
? complete_all+0x60/0x60
__lock_buffer+0x3f/0x50
do_get_write_access+0x2ad/0x550
jbd2_journal_get_write_access+0x51/0x80
__ext4_journal_get_write_access+0xf9/0x1a0
ext4_ext_get_access.isra.32+0x34/0x50
ext4_ext_remove_space+0x59f/0x1830
? ext4_ext_truncate+0xa2/0xf0
? ext4_ext_truncate+0x5e/0xf0
ext4_ext_truncate+0xd6/0xf0
ext4_truncate+0x46a/0x660
ext4_setattr+0x6db/0xa40
notify_change+0x3e5/0x540
do_truncate+0x7b/0xc0
? do_truncate+0x7b/0xc0
do_sys_ftruncate+0x164/0x2f0
__x64_sys_ftruncate+0x1b/0x20
do_syscall_64+0x3f/0x90
entry_SYSCALL_64_after_hwframe+0x46/0xb0
RIP: 0033:0x7f3f9dae3f7b
RSP: 002b:00007ffe06430318 EFLAGS: 00000202 ORIG_RAX: 000000000000004d
RAX: ffffffffffffffda RBX: 00000000000001a5 RCX: 00007f3f9dae3f7b
RDX: 00000000000001a5 RSI: 00000000000d2800 RDI: 0000000000000003
RBP: 00000000000001a5 R08: 00007f3f9dbce090 R09: 00007f3f9dbce0a0
R10: 0000000000000008 R11: 0000000000000202 R12: 0000000000000000
R13: 0000000000000207 R14: 0000000000000043 R15: 0000000000000003
</TASK>
5 locks held by ftest01/818:
#0: ffff957b9c9b74e8 (sb_writers#3){.+.+}-{0:0}, at: __x64_sys_ftruncate+0x1b/0x20
#1: ffff957c2a679b00 (&sb->s_type->i_mutex_key#10){+.+.}-{3:3}, at: do_truncate+0x6a/0xc0
#2: ffff957c2a679cc0 (mapping.invalidate_lock){++++}-{3:3}, at: ext4_setattr+0x39a/0xa40
#3: ffff957b9c9b5be8 (jbd2_handle){++++}-{0:0}, at: start_this_handle+0x13e/0x5f0
#4: ffff957c2a679958 (&ei->i_data_sem){++++}-{3:3}, at: ext4_truncate+0x443/0x660
task:ftest01 state:D stack: 0 pid: 819 ppid: 816 flags:0x00004002
Call Trace:
<TASK>
__schedule+0x329/0xae0
? __x64_sys_tee+0xd0/0xd0
? __x64_sys_tee+0xd0/0xd0
schedule+0x48/0xc0
wb_wait_for_completion+0x62/0x90
? schedstat_stop+0x10/0x10
sync_inodes_sb+0xdd/0x460
? __this_cpu_preempt_check+0x13/0x20
? __x64_sys_tee+0xd0/0xd0
sync_inodes_one_sb+0x15/0x20
iterate_supers+0x94/0x100
ksys_sync+0x42/0xa0
__do_sys_sync+0xe/0x20
do_syscall_64+0x3f/0x90
entry_SYSCALL_64_after_hwframe+0x46/0xb0
RIP: 0033:0x7f3f9dae2cdb
RSP: 002b:00007ffe06430318 EFLAGS: 00000246 ORIG_RAX: 00000000000000a2
RAX: ffffffffffffffda RBX: 0000000000000800 RCX: 00007f3f9dae2cdb
RDX: 0000000000000040 RSI: 0000561f57de52f0 RDI: 0000561f57de5340
RBP: 00000000000c2800 R08: 00000000fffff800 R09: 0000000000004000
R10: 0000000000000010 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000207 R14: 0000000000000075 R15: 0000000000000003
</TASK>
1 lock held by ftest01/819:
#0: ffff957b9c9b70d0 (&type->s_umount_key#34){++++}-{3:3}, at: iterate_supers+0x7d/0x100
task:ftest01 state:D stack: 0 pid: 820 ppid: 816 flags:0x00004002
Call Trace:
<TASK>
__schedule+0x329/0xae0
? __x64_sys_tee+0xd0/0xd0
? __x64_sys_tee+0xd0/0xd0
schedule+0x48/0xc0
wb_wait_for_completion+0x62/0x90
? schedstat_stop+0x10/0x10
sync_inodes_sb+0xdd/0x460
? __this_cpu_preempt_check+0x13/0x20
? __x64_sys_tee+0xd0/0xd0
sync_inodes_one_sb+0x15/0x20
iterate_supers+0x94/0x100
ksys_sync+0x42/0xa0
__do_sys_sync+0xe/0x20
do_syscall_64+0x3f/0x90
entry_SYSCALL_64_after_hwframe+0x46/0xb0
RIP: 0033:0x7f3f9dae2cdb
RSP: 002b:00007ffe06430318 EFLAGS: 00000246 ORIG_RAX: 00000000000000a2
RAX: ffffffffffffffda RBX: 0000000000000800 RCX: 00007f3f9dae2cdb
RDX: 0000000000000040 RSI: 0000561f57de52f0 RDI: 0000561f57de5340
RBP: 000000000006a800 R08: 00000000fffff800 R09: 0000000000004000
R10: 0000000000000010 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000207 R14: 0000000000000069 R15: 0000000000000003
</TASK>
1 lock held by ftest01/820:
#0: ffff957b9c9b70d0 (&type->s_umount_key#34){++++}-{3:3}, at: iterate_supers+0x7d/0x100
task:ftest01 state:D stack: 0 pid: 821 ppid: 816 flags:0x00000002
Call Trace:
<TASK>
__schedule+0x329/0xae0
? bit_wait+0x60/0x60
schedule+0x48/0xc0
io_schedule+0x16/0x40
bit_wait_io+0x11/0x60
__wait_on_bit_lock+0x5e/0xb0
out_of_line_wait_on_bit_lock+0x93/0xb0
? complete_all+0x60/0x60
__lock_buffer+0x3f/0x50
do_get_write_access+0x2ad/0x550
jbd2_journal_get_write_access+0x51/0x80
__ext4_journal_get_write_access+0xf9/0x1a0
ext4_ext_get_access.isra.32+0x34/0x50
ext4_ext_remove_space+0x59f/0x1830
? ext4_ext_truncate+0xa2/0xf0
? ext4_ext_truncate+0x5e/0xf0
ext4_ext_truncate+0xd6/0xf0
ext4_truncate+0x46a/0x660
ext4_setattr+0x6db/0xa40
notify_change+0x3e5/0x540
do_truncate+0x7b/0xc0
? do_truncate+0x7b/0xc0
do_sys_ftruncate+0x164/0x2f0
__x64_sys_ftruncate+0x1b/0x20
do_syscall_64+0x3f/0x90
entry_SYSCALL_64_after_hwframe+0x46/0xb0
RIP: 0033:0x7f3f9dae3f7b
RSP: 002b:00007ffe06430318 EFLAGS: 00000202 ORIG_RAX: 000000000000004d
RAX: ffffffffffffffda RBX: 000000000000016a RCX: 00007f3f9dae3f7b
RDX: 000000000000016a RSI: 00000000000b5000 RDI: 0000000000000003
RBP: 000000000000016a R08: 00007f3f9dbce080 R09: 00007f3f9dbce0a0
R10: 0000000000000002 R11: 0000000000000202 R12: 0000000000000000
R13: 0000000000000207 R14: 0000000000000040 R15: 0000000000000003
</TASK>
5 locks held by ftest01/821:
#0: ffff957b9c9b74e8 (sb_writers#3){.+.+}-{0:0}, at: __x64_sys_ftruncate+0x1b/0x20
#1: ffff957c2a67bd20 (&sb->s_type->i_mutex_key#10){+.+.}-{3:3}, at: do_truncate+0x6a/0xc0
#2: ffff957c2a67bee0 (mapping.invalidate_lock){++++}-{3:3}, at: ext4_setattr+0x39a/0xa40
#3: ffff957b9c9b5be8 (jbd2_handle){++++}-{0:0}, at: start_this_handle+0x13e/0x5f0
#4: ffff957c2a67bb78 (&ei->i_data_sem){++++}-{3:3}, at: ext4_truncate+0x443/0x660
--------------------------------------
Reproducer:
Get latest preempt-rt kernel:
git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git
checkout v5.19-rc3-rt5 (or linux-5.19.y-rt branch)
make defconfig
manually enable CONFIG_EXPERT and then CONFIG_PREEMPT_RT
make oldconfig and take defaults for other options hidden behind EXPERT
can optionally enable LOCKDEP options so sysrq-d works.
Get LTP ; build + install
https://github.com/linux-test-project/ltp.git
Boot and then run reproducer:
dd if=/dev/zero of=/root/tmp_mount_file bs=1M count=512
mkfs.ext4 -F /root/tmp_mount_file
mount /root/tmp_mount_file /mnt/
/opt/ltp/runltp -f fs -s ftest01 -d /mnt/
umount /mnt
loop device isn't strictly necessary but contains fs fallout when
inevitable reboot is required to get rid of D state processes.
Test completes on v5.10-rt, v5.12-rt - should take about 2-3m total.
Test hangs forever on v5.15-rt and latest v5.19-rc-rt. Test threads
in D state - any other i/o directed at /mnt also hangs in D state.
