Re: blkdev_get deadlock

[Bart Van Assche]

On 5/3/19 10:47 AM, Evan Green wrote:
> Hey blockies,
  ^^^^^^^^^^^^

That's the weirdest greeting I have encountered so far on the
linux-block mailing list.

> I'm seeing a hung task in the kernel, and I wanted to share it in case
> it's a known issue. I'm still trying to wrap my head around the stacks
> myself. This is our Chrome OS 4.19 kernel, which is admittedly not
> 100% vanilla mainline master, but we try to keep it pretty close.
> 
> I can reproduce this reliably within our chrome OS installer, where
> it's trying to dd from my system disk (NVMe) to a loop device backed
> by a removable UFS card (4kb sectors) in a USB dongle.

Although this is not the only possible cause such hangs are often caused
by a block driver or SCSI LLD not completing a request. A list of
pending requests can be obtained e.g. by running the attached script.

Bart.
#!/bin/bash

show_state() {
    local a dev=$1

    for a in device/state queue/scheduler; do
	[ -e "$dev/$a" ] && grep -aH . "$dev/$a"
    done
}

if [ -e /sys/kernel/debug/block ]; then
    devs=($(cd /sys/kernel/debug/block && echo ./*))
else
    devs=($(cd /sys/class/block && echo ./*))
fi

cd /sys/class/block || exit $?
for dev in "${devs[@]}"; do
    dev="${dev#./}"
    echo "$dev"
    pending=0
    if [ -e "$dev/mq" ]; then
	for f in "$dev"/mq/*/{pending,*/rq_list}; do
	    [ -e "$f" ] || continue
	    if { read -r line1 && read -r line2; } <"$f"; then
		echo "$f"
		echo "$line1 $line2" >/dev/null
		head -n 9 "$f"
		((pending++))
	    fi
	done
    fi
    (
	busy=0
	cd /sys/kernel/debug/block >&/dev/null &&
	    { grep -aH . $dev/requeue_list; true; } &&
	    for d in "$dev"/mq/hctx* "$dev"/hctx*; do
		[ ! -d "$d" ] && continue
		{ [ ! -e "$d/tags" ] ||
		      grep -q '^busy=0$' "$d/tags"; } &&
		    { [ ! -e "$d/sched_tags" ] ||
			  [ "$(<"$d/sched_tags")" = "" ] ||
			  grep -q '^busy=0$' "$d/sched_tags"; } && continue
		((busy++))
	        for f in "$d"/{active,busy,dispatch,flags,requeue_list,sched_tags,state,tags*,cpu*/rq_list,sched/*rqs}; do
		    [ -e "$f" ] && grep -aH . "$f"
		done
	    done
	exit $busy
    )
    pending=$((pending+$?))
    if [ "$pending" -gt 0 ]; then
	(
	    cd /sys/kernel/debug/block >&/dev/null &&
		if [ -e "$dev/mq/state" ]; then
		    grep -aH . "$dev/mq/state"
		else
		    grep -aH . "$dev/state"
		fi
	)
	show_state "$dev"
    fi
done