Re: [Linux kernel bug] INFO: task hung in blk_mq_get_tag

[Sam Sun]

On Tue, May 14, 2024 at 10:46 PM Tetsuo Handa
<penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On 2024/05/14 21:07, Sam Sun wrote:
> > I tried to run
> >
> > # echo 0 > /proc/sys/kernel/hung_task_all_cpu_backtrace
> >
> > before running the reproducer, the kernel stops panic. But still, even
> > if I terminate the execution of the reproducer, kernel continues
> > dumping task hung logs. After setting bung_task_all_cpu_backtrace back
> > to 1, it panic immediately during next dump. So I guess it is still a
> > task hung instead of general protection fault.
>
> What execute_one() in the reproducer is doing is only
>
>   int fd1 = open("/dev/sg0", O_RDONLY);
>   int fd2 = open("/sys/module/sg/parameters/allow_dio", O_RDWR);
>   write(fd2, "100\0", 4); // returns 4
>   ioctl(fd1, FIBMAP, 0x20000140); // returns 2
>
> . But your hung task report includes device rescan sequence.
>
>  schedule+0x147/0x310 kernel/sched/core.c:6838
>  io_schedule+0x87/0x100 kernel/sched/core.c:9044
>  blk_mq_get_tag+0x509/0xba0 block/blk-mq-tag.c:187
>  __blk_mq_alloc_requests+0xbc1/0x1710 block/blk-mq.c:499
>  blk_mq_alloc_request+0x513/0xbc0 block/blk-mq.c:599
>  scsi_alloc_request drivers/scsi/scsi_lib.c:1229 [inline]
>  scsi_execute_cmd+0x17a/0x1140 drivers/scsi/scsi_lib.c:304
>  scsi_vpd_inquiry drivers/scsi/scsi.c:312 [inline]
>  scsi_get_vpd_size+0x2e3/0x6b0 drivers/scsi/scsi.c:363
>  scsi_get_vpd_buf+0x89/0x460 drivers/scsi/scsi.c:433
>  scsi_attach_vpd+0xdc/0x5e0 drivers/scsi/scsi.c:501
>  scsi_rescan_device+0xd8/0x290 drivers/scsi/scsi_scan.c:1698
>  ata_scsi_dev_rescan+0x1fe/0x3c0 drivers/ata/libata-scsi.c:4764
>  process_one_work kernel/workqueue.c:3254 [inline]
>
> Something is triggering this sequence, and writing to allow_dio interface
> confuses the "if (sg_allow_dio && ...)" path in sg_start_req()
> in drivers/scsi/sg.c ?
>
> What happens if you disable
>
>   sysfd = write(sysfd, input, hash - input + 1);
>
> line (i.e. stop updating sg_allow_dio value) in the reproducer?
>

I tried to change the value of /sys/module/sg/parameters/allow_dio to
0 and remove write() call, both still triggers task hang report and
kernel panic. I think this write is not the call crashing the kernel.