Re: INFO: task hung in blk_queue_enter

Tetsuo Handa
2018-05-15T11:45:26Z

I managed to obtain SysRq-t when khungtaskd fires using debug printk()
( https://groups.google.com/forum/#!topic/syzkaller-bugs/OTuOsVebAiE ).
Only 4 threads shown below seems to be relevant to this problem.

[ 246.929688] task PC stack pid father
[ 249.888937] jbd2/sda1-8 D17736 2271 2 0x80000000
[ 249.894586] Call Trace:
[ 249.897193] __schedule+0x801/0x1e30
[ 249.900954] schedule+0xef/0x430
[ 249.904356] io_schedule+0x1c/0x70
[ 249.907935] bit_wait_io+0x18/0x90
[ 249.911492] __wait_on_bit+0xb3/0x130
[ 249.915313] out_of_line_wait_on_bit+0x204/0x3a0
[ 249.920105] __wait_on_buffer+0x76/0x90
[ 249.924102] jbd2_journal_commit_transaction+0x655b/0x8c18
[ 249.929893] kjournald2+0x26c/0xb30
[ 249.933579] kthread+0x345/0x410
[ 249.936966] ret_from_fork+0x3a/0x50
[ 254.408972] syz-executor7 D18976 5010 4828 0x00000004
[ 254.414639] Call Trace:
[ 254.417263] __schedule+0x801/0x1e30
[ 254.421070] schedule+0xef/0x430
[ 254.424467] blk_queue_enter+0x8da/0xdf0
[ 254.428584] generic_make_request+0x144/0x1510
[ 254.433217] blk_queue_split+0x374/0x2090
[ 254.437425] blk_mq_make_request+0x2d0/0x25c0
[ 254.442004] generic_make_request+0x795/0x1510
[ 254.446663] submit_bio+0xba/0x460
[ 254.451104] mpage_readpages+0x6d7/0x970
[ 254.455224] blkdev_readpages+0x2c/0x40
[ 254.459220] __do_page_cache_readahead+0x79a/0xdc0
[ 254.464205] ondemand_readahead+0x550/0xc40
[ 254.468559] page_cache_sync_readahead+0xd1/0x110
[ 254.473430] generic_file_read_iter+0x1a74/0x2f00
[ 254.478423] blkdev_read_iter+0x120/0x190
[ 254.482594] generic_file_splice_read+0x552/0x910
[ 254.487484] do_splice_to+0x12e/0x190
[ 254.491311] splice_direct_to_actor+0x268/0x8d0
[ 254.496017] do_splice_direct+0x2cc/0x400
[ 254.500224] do_sendfile+0x60f/0xe00
[ 254.503971] __x64_sys_sendfile64+0x155/0x240
[ 254.508507] do_syscall_64+0x1b1/0x800
[ 254.512431] entry_SYSCALL_64_after_hwframe+0x49/0xbe
[ 254.568779] syz-executor7 D25408 5021 4828 0x00000004
[ 254.574429] Call Trace:
[ 254.577125] __schedule+0x801/0x1e30
[ 254.580882] schedule+0xef/0x430
[ 254.584297] blk_mq_freeze_queue_wait+0x1ce/0x460
[ 254.589192] blk_freeze_queue+0x4a/0x80
[ 254.593209] blk_mq_freeze_queue+0x15/0x20
[ 254.597464] loop_clr_fd+0x226/0xb80
[ 254.601208] lo_ioctl+0x642/0x2130
[ 254.604774] blkdev_ioctl+0x9b6/0x2020
[ 254.608715] block_ioctl+0xee/0x130
[ 254.612362] do_vfs_ioctl+0x1cf/0x16a0
[ 254.616294] ksys_ioctl+0xa9/0xd0
[ 254.619770] __x64_sys_ioctl+0x73/0xb0
[ 254.623674] do_syscall_64+0x1b1/0x800
[ 254.627596] entry_SYSCALL_64_after_hwframe+0x49/0xbe
[ 255.022839] 1 lock held by syz-executor7/5021:
[ 255.028209] #0: (ptrval) (&lo->lo_ctl_mutex/1){+.+.}, at: lo_ioctl+0x8d/0x2130
[ 254.719133] blkid D23880 5071 2713 0x00000000
[ 254.724791] Call Trace:
[ 254.727402] __schedule+0x801/0x1e30
[ 254.731159] schedule+0xef/0x430
[ 254.734562] schedule_preempt_disabled+0x10/0x20
[ 254.739333] __mutex_lock+0xe38/0x17f0
[ 254.743280] mutex_lock_killable_nested+0x16/0x20
[ 254.748135] lo_ioctl+0x8d/0x2130
[ 254.751611] blkdev_ioctl+0x9b6/0x2020
[ 254.755550] block_ioctl+0xee/0x130
[ 254.759200] do_vfs_ioctl+0x1cf/0x16a0
[ 254.763123] ksys_ioctl+0xa9/0xd0
[ 254.766601] __x64_sys_ioctl+0x73/0xb0
[ 254.770506] do_syscall_64+0x1b1/0x800
[ 254.774425] entry_SYSCALL_64_after_hwframe+0x49/0xbe
[ 255.036399] 1 lock held by blkid/5071:
[ 255.041075] #0: (ptrval) (&lo->lo_ctl_mutex/1){+.+.}, at: lo_ioctl+0x8d/0x2130

syz-executor7(PID=5021) is doing ioctl(LOOP_CLR_FD).

blkid(PID=5071) is doing ioctl() but is blocked on lo_ctl_mutex which is
held by syz-executor7(PID=5021). Therefore, I assume that blkid(PID=5071)
is irrelevant for this hung up problem.

syz-executor7(PID=5021) is stuck at blk_mq_freeze_queue_wait() from
blk_freeze_queue() from blk_mq_freeze_queue() from loop_clr_fd().

/*
* Guarantee no request is in use, so we can change any data structure of
* the queue afterward.
*/
void blk_freeze_queue(struct request_queue *q)
{
/*
* In the !blk_mq case we are only calling this to kill the
* q_usage_counter, otherwise this increases the freeze depth
* and waits for it to return to zero. For this reason there is
* no blk_unfreeze_queue(), and blk_freeze_queue() is not
* exported to drivers as the only user for unfreeze is blk_mq.
*/
blk_freeze_queue_start(q);
if (!q->mq_ops)
blk_drain_queue(q);
blk_mq_freeze_queue_wait(q);
}

q->mq_freeze_depth is incremented at blk_freeze_queue_start() and
the caller waits until q->q_usage_counter becomes 0.

void blk_freeze_queue_start(struct request_queue *q)
{
int freeze_depth;

freeze_depth = atomic_inc_return(&q->mq_freeze_depth);
if (freeze_depth == 1) {
percpu_ref_kill(&q->q_usage_counter);
if (q->mq_ops)
blk_mq_run_hw_queues(q, false);
}
}

void blk_mq_freeze_queue_wait(struct request_queue *q)
{
wait_event(q->mq_freeze_wq, percpu_ref_is_zero(&q->q_usage_counter));
}

I couldn't check whether freeze_depth in blk_freeze_queue_start() was 1,
but presumably q->mq_freeze_depth > 0 because syz-executor7(PID=5010) is
stuck at wait_event() in blk_queue_enter().

syz-executor7 D18976 5010 4828 0x00000004
Call Trace:
context_switch kernel/sched/core.c:2859 [inline]
__schedule+0x801/0x1e30 kernel/sched/core.c:3501
schedule+0xef/0x430 kernel/sched/core.c:3545
blk_queue_enter+0x8da/0xdf0 block/blk-core.c:953
generic_make_request+0x144/0x1510 block/blk-core.c:2395
blk_queue_split+0x374/0x2090 block/blk-merge.c:215
blk_mq_make_request+0x2d0/0x25c0 block/blk-mq.c:1861
generic_make_request+0x795/0x1510 block/blk-core.c:2460
submit_bio+0xba/0x460 block/blk-core.c:2568
mpage_bio_submit fs/mpage.c:66 [inline]
mpage_readpages+0x6d7/0x970 fs/mpage.c:393
blkdev_readpages+0x2c/0x40 fs/block_dev.c:577
read_pages mm/readahead.c:121 [inline]
__do_page_cache_readahead+0x79a/0xdc0 mm/readahead.c:199
ra_submit mm/internal.h:66 [inline]
ondemand_readahead+0x550/0xc40 mm/readahead.c:478
page_cache_sync_readahead+0xd1/0x110 mm/readahead.c:510
generic_file_buffered_read mm/filemap.c:2092 [inline]
generic_file_read_iter+0x1a74/0x2f00 mm/filemap.c:2362
blkdev_read_iter+0x120/0x190 fs/block_dev.c:1930
call_read_iter include/linux/fs.h:1778 [inline]
generic_file_splice_read+0x552/0x910 fs/splice.c:307
do_splice_to+0x12e/0x190 fs/splice.c:880
splice_direct_to_actor+0x268/0x8d0 fs/splice.c:952
do_splice_direct+0x2cc/0x400 fs/splice.c:1061
do_sendfile+0x60f/0xe00 fs/read_write.c:1440
__do_sys_sendfile64 fs/read_write.c:1495 [inline]
__se_sys_sendfile64 fs/read_write.c:1487 [inline]
__x64_sys_sendfile64+0x155/0x240 fs/read_write.c:1487
do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
entry_SYSCALL_64_after_hwframe+0x49/0xbe

Since flags == 0, preempt == false. Since stuck at wait_event(), success == false.
Thus, atomic_read(&q->mq_freeze_depth) > 0 if blk_queue_dying(q) == false. And I
guess blk_queue_dying(q) == false because we are just trying to freeze/unfreeze.

/**
* blk_queue_enter() - try to increase q->q_usage_counter
* @q: request queue pointer
* @flags: BLK_MQ_REQ_NOWAIT and/or BLK_MQ_REQ_PREEMPT
*/
int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
{
const bool preempt = flags & BLK_MQ_REQ_PREEMPT;

while (true) {
bool success = false;

rcu_read_lock();
if (percpu_ref_tryget_live(&q->q_usage_counter)) {
/*
* The code that sets the PREEMPT_ONLY flag is
* responsible for ensuring that that flag is globally
* visible before the queue is unfrozen.
*/
if (preempt || !blk_queue_preempt_only(q)) {
success = true;
} else {
percpu_ref_put(&q->q_usage_counter);
}
}
rcu_read_unlock();

if (success)
return 0;

if (flags & BLK_MQ_REQ_NOWAIT)
return -EBUSY;

/*
* read pair of barrier in blk_freeze_queue_start(),
* we need to order reading __PERCPU_REF_DEAD flag of
* .q_usage_counter and reading .mq_freeze_depth or
* queue dying flag, otherwise the following wait may
* never return if the two reads are reordered.
*/
smp_rmb();

wait_event(q->mq_freeze_wq,
(atomic_read(&q->mq_freeze_depth) == 0 &&
(preempt || !blk_queue_preempt_only(q))) ||
blk_queue_dying(q));
if (blk_queue_dying(q))
return -ENODEV;
}
}

jbd2/sda1-8(PID=2271) is stuck waiting for journal commit operation.
I don't know how this thread is involved to this problem.

jbd2/sda1-8 D17736 2271 2 0x80000000
Call Trace:
context_switch kernel/sched/core.c:2859 [inline]
__schedule+0x801/0x1e30 kernel/sched/core.c:3501
schedule+0xef/0x430 kernel/sched/core.c:3545
io_schedule+0x1c/0x70 kernel/sched/core.c:5165
bit_wait_io+0x18/0x90 kernel/sched/wait_bit.c:207
__wait_on_bit+0xb3/0x130 kernel/sched/wait_bit.c:48
out_of_line_wait_on_bit+0x204/0x3a0 kernel/sched/wait_bit.c:63
wait_on_bit_io include/linux/wait_bit.h:101 [inline]
__wait_on_buffer+0x76/0x90 fs/buffer.c:118
wait_on_buffer include/linux/buffer_head.h:356 [inline]
journal_wait_on_commit_record fs/jbd2/commit.c:174 [inline]
jbd2_journal_commit_transaction+0x655b/0x8c18 fs/jbd2/commit.c:865
kjournald2+0x26c/0xb30 fs/jbd2/journal.c:229
kthread+0x345/0x410 kernel/kthread.c:240
ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:412

Can we find the culprit from these hints?