Re: [PATCH 2/2] blk-mq: ensure a q_usage_counter reference is held when splitting bios

[Jens Axboe <axboe@kernel.dk>]

On 1/11/24 6:57 AM, Christoph Hellwig wrote:
> q_usage_counter is the only thing preventing us from the limits changing
> under us in __bio_split_to_limits, but blk_mq_submit_bio doesn't hold it.
> 
> Change __submit_bio to always acquire the q_usage_counter counter before
> branching out into bio vs request based helper, and let blk_mq_submit_bio
> tell it if it consumed the reference by handing it off to the request.

This causes hangs for me on shutdown/reset:

[   56.146054] sd 6:0:0:0: [sdb] Synchronizing SCSI cache
[   56.147739] sd 6:0:0:0: [sdb] Stopping disk
[   56.148976] sd 0:0:0:0: [sda] Synchronizing SCSI cache
[   56.150803] sd 0:0:0:0: [sda] Stopping disk
[   75.549201] INFO: task systemd-shutdow:1 blocked for more than 15 seconds.
[   75.549636]       Not tainted 6.7.0-rc5-00173-g34d71db9cce2 #4540
[   75.549977] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[   75.550401] task:systemd-shutdow state:D stack:0     pid:1     tgid:1     ppid:0      flags:0x00000004
[   75.550900] Call trace:
[   75.551042]  __switch_to+0x114/0x150
[   75.551253]  __schedule+0x510/0x10d4
[   75.551451]  schedule+0x7c/0x1ac
[   75.551635]  schedule_timeout+0xe8/0x1a4
[   75.551857]  blk_mq_freeze_queue_wait_timeout+0xf4/0x18c
[   75.552157]  nvme_wait_freeze_timeout+0x68/0xa4
[   75.552503]  nvme_dev_disable+0x35c/0x374
[   75.552734]  nvme_shutdown+0x34/0x40
[   75.552956]  pci_device_shutdown+0x48/0x54
[   75.553184]  device_shutdown+0x1c4/0x314
[   75.553403]  kernel_power_off+0x40/0x88

which seems to indicate that a reference is being leaked. Haven't poked
any further at it, I'll drop these two for now.

-- 
Jens Axboe