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SubjectRe: INFO: task hung in __blkdev_get
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I tried the reproducer in my environment. The reproducer can trivially
reproduce a hung up. If the bug I'm observing is what the syzbot is
reporting (I ran the reproducer using init= kernel command line option),
the reason __blkdev_get() is blocked waiting for bdev->bd_mutex is that
an exiting thread cannot release bdev->bd_mutex held at __blkdev_put()
because that thread is unable to return from wait_event_interruptible()
in blk_queue_enter(). (By the way, since the exiting task is sleeping in
interruptible state, khungtaskd does not help; we need to use SysRq-t.)

----------------------------------------
[ 200.731135] a.out S 0 447 446 0x80000002
[ 200.732646] Call Trace:
[ 200.733566] ? __schedule+0x2a4/0x9f0
[ 200.734764] ? _raw_spin_unlock_irqrestore+0x40/0x50
[ 200.736251] schedule+0x34/0x80
[ 200.737353] schedule_timeout+0x1cd/0x530
[ 200.738595] ? collect_expired_timers+0xa0/0xa0
[ 200.739921] ? blk_queue_enter+0x7d/0x550
[ 200.741500] blk_queue_enter+0x275/0x550
[ 200.743017] ? wait_woken+0x80/0x80
[ 200.744273] generic_make_request+0xe3/0x2a0
[ 200.745621] ? submit_bio+0x67/0x130
[ 200.746831] submit_bio+0x67/0x130
[ 200.747915] ? guard_bio_eod+0xae/0x200
[ 200.749177] submit_bh_wbc+0x161/0x190
[ 200.750326] __block_write_full_page+0x15c/0x3c0
[ 200.751678] ? check_disk_change+0x60/0x60
[ 200.752923] __writepage+0x11/0x50
[ 200.754023] write_cache_pages+0x1ea/0x530
[ 200.755575] ? __test_set_page_writeback+0x440/0x440
[ 200.757039] ? __lock_acquire+0x38f/0x1a10
[ 200.758718] generic_writepages+0x5f/0xa0
[ 200.760128] ? do_writepages+0x12/0x50
[ 200.761281] do_writepages+0x12/0x50
[ 200.762369] __filemap_fdatawrite_range+0xc3/0x100
[ 200.763719] ? __mutex_lock+0x72/0x950
[ 200.764876] filemap_write_and_wait+0x25/0x60
[ 200.766168] __blkdev_put+0x71/0x200
[ 200.767261] blkdev_close+0x1c/0x20
[ 200.768333] __fput+0x95/0x1d0
[ 200.769339] task_work_run+0x84/0xa0
[ 200.770767] do_exit+0x301/0xbf0
[ 200.771877] ? __do_page_fault+0x2ca/0x510
[ 200.773086] do_group_exit+0x38/0xb0
[ 200.774260] SyS_exit_group+0xb/0x10
[ 200.776094] do_syscall_64+0x68/0x210
[ 200.777486] entry_SYSCALL_64_after_hwframe+0x42/0xb7

[ 200.815616] Showing all locks held in the system:
[ 200.817791] 2 locks held by kworker/0:1/38:
[ 200.819125] #0: 0000000057489670 ((wq_completion)"events_freezable_power_efficient"){+.+.}, at: process_one_work+0x1e2/0x690
[ 200.821878] #1: 00000000c9c74590 ((work_completion)(&(&ev->dwork)->work)){+.+.}, at: process_one_work+0x1e2/0x690
[ 200.824690] 1 lock held by a.out/447:
[ 200.826448] #0: 00000000bedfcca8 (&bdev->bd_mutex){+.+.}, at: __blkdev_put+0x3c/0x200


[ 270.495110] a.out S 0 447 446 0x80000002
[ 270.496770] Call Trace:
[ 270.497779] ? __schedule+0x2a4/0x9f0
[ 270.498930] ? _raw_spin_unlock_irqrestore+0x40/0x50
[ 270.500433] schedule+0x34/0x80
[ 270.501513] schedule_timeout+0x1cd/0x530
[ 270.502773] ? collect_expired_timers+0xa0/0xa0
[ 270.504125] ? blk_queue_enter+0x7d/0x550
[ 270.505706] blk_queue_enter+0x275/0x550
[ 270.507352] ? wait_woken+0x80/0x80
[ 270.508614] generic_make_request+0xe3/0x2a0
[ 270.509913] ? submit_bio+0x67/0x130
[ 270.511876] submit_bio+0x67/0x130
[ 270.513160] ? guard_bio_eod+0xae/0x200
[ 270.514430] submit_bh_wbc+0x161/0x190
[ 270.515691] __block_write_full_page+0x15c/0x3c0
[ 270.517024] ? check_disk_change+0x60/0x60
[ 270.518301] __writepage+0x11/0x50
[ 270.519420] write_cache_pages+0x1ea/0x530
[ 270.520691] ? __test_set_page_writeback+0x440/0x440
[ 270.522118] ? __lock_acquire+0x38f/0x1a10
[ 270.523377] generic_writepages+0x5f/0xa0
[ 270.524624] ? do_writepages+0x12/0x50
[ 270.525974] do_writepages+0x12/0x50
[ 270.527222] __filemap_fdatawrite_range+0xc3/0x100
[ 270.529115] ? __mutex_lock+0x72/0x950
[ 270.530378] filemap_write_and_wait+0x25/0x60
[ 270.531741] __blkdev_put+0x71/0x200
[ 270.532904] blkdev_close+0x1c/0x20
[ 270.534074] __fput+0x95/0x1d0
[ 270.535139] task_work_run+0x84/0xa0
[ 270.536331] do_exit+0x301/0xbf0
[ 270.537458] ? __do_page_fault+0x2ca/0x510
[ 270.538717] do_group_exit+0x38/0xb0
[ 270.539830] SyS_exit_group+0xb/0x10
[ 270.540963] do_syscall_64+0x68/0x210
[ 270.542097] entry_SYSCALL_64_after_hwframe+0x42/0xb7

[ 270.582465] Showing all locks held in the system:
[ 270.584584] 2 locks held by kworker/1:2/136:
[ 270.585939] #0: 0000000057489670 ((wq_completion)"events_freezable_power_efficient"){+.+.}, at: process_one_work+0x1e2/0x690
[ 270.589850] #1: 00000000e793cf79 ((work_completion)(&(&ev->dwork)->work)){+.+.}, at: process_one_work+0x1e2/0x690
[ 270.592813] 1 lock held by a.out/447:
[ 270.594237] #0: 00000000bedfcca8 (&bdev->bd_mutex){+.+.}, at: __blkdev_put+0x3c/0x200
----------------------------------------

I checked variables using below patch, but no variable changed over time.

----------------------------------------
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -857,10 +857,26 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
*/
smp_rmb();

- ret = wait_event_interruptible(q->mq_freeze_wq,
- (atomic_read(&q->mq_freeze_depth) == 0 &&
- (preempt || !blk_queue_preempt_only(q))) ||
- blk_queue_dying(q));
+ while (1) {
+ ret = wait_event_interruptible_timeout
+ (q->mq_freeze_wq,
+ (atomic_read(&q->mq_freeze_depth) == 0 &&
+ (preempt || !blk_queue_preempt_only(q))) ||
+ blk_queue_dying(q), 5 * HZ);
+ if (ret < 0) {
+ break;
+ } else if (ret == 0) {
+ printk("q->mq_freeze_depth=%d preempt=%u "
+ "blk_queue_preempt_only(q)=%u "
+ "blk_queue_dying(q)=%u\n",
+ atomic_read(&q->mq_freeze_depth),
+ preempt, blk_queue_preempt_only(q),
+ blk_queue_dying(q));
+ } else {
+ ret = 0;
+ break;
+ }
+ }
if (blk_queue_dying(q))
return -ENODEV;
if (ret)
----------------------------------------
[ 28.090809] loop: module loaded
[ 43.488409] q->mq_freeze_depth=1 preempt=0 blk_queue_preempt_only(q)=0 blk_queue_dying(q)=0
[ 48.608715] q->mq_freeze_depth=1 preempt=0 blk_queue_preempt_only(q)=0 blk_queue_dying(q)=0
[ 53.728281] q->mq_freeze_depth=1 preempt=0 blk_queue_preempt_only(q)=0 blk_queue_dying(q)=0
[ 58.848387] q->mq_freeze_depth=1 preempt=0 blk_queue_preempt_only(q)=0 blk_queue_dying(q)=0
(...snipped...)
[ 289.248694] q->mq_freeze_depth=1 preempt=0 blk_queue_preempt_only(q)=0 blk_queue_dying(q)=0
[ 294.368484] q->mq_freeze_depth=1 preempt=0 blk_queue_preempt_only(q)=0 blk_queue_dying(q)=0
[ 299.488896] q->mq_freeze_depth=1 preempt=0 blk_queue_preempt_only(q)=0 blk_queue_dying(q)=0
[ 304.608632] q->mq_freeze_depth=1 preempt=0 blk_queue_preempt_only(q)=0 blk_queue_dying(q)=0

This means that we deadlocked there.

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