Re: [PATCH] block: Fix bug in runtime-resume handling

Martin Kepplinger
2020-08-29T16:34:04Z

On 29.08.20 17:26, Alan Stern wrote:
> On Sat, Aug 29, 2020 at 09:24:30AM +0200, Martin Kepplinger wrote:
>> On 27.08.20 22:29, Alan Stern wrote:
>>> Instead, look at sd_resume(). That routine calls __scsi_execute()
>>> indirectly through sd_start_stop_device(), and the only reason it does
>>> this is because the sdkp->device->manage_start_stop flag is set. You
>>> ought to be able to clear this flag in sysfs, by writing to
>>> /sys/block/sda/device/scsi_disk/*/manage_start_stop. If you do this
>>> before allowing the card reader to go into runtime suspend, does it then
>>> resume okay?
>>
>> manage_start_stop in sysfs is 0 here.
>
> Hmmm. I'm wondering about something you wrote back in June
> (https://marc.info/?l=linux-scsi&m=159345778431615&w=2):
>
> blk_queue_enter() always - especially when sd is runtime
> suspended and I try to mount as above - sets success to be true
> for me, so never continues down to bkl_pm_request_resume(). All
> I see is "PM: Removing info for No Bus:sda1".
>
> blk_queue_enter() would always set success to be true because pm
> (derived from the BLK_MQ_REQ_PREEMPT flag) is true. But why was the
> BLK_MQ_REQ_PREEMPT flag set? In other words, where was
> blk_queue_enter() called from?
>
> Can you get a stack trace (i.e., call dump_stack()) at exactly this
> point, that is, when pm is true and q->rpm_status is RPM_SUSPENDED? Or
> do you already know the answer?
>
>

I reverted any scsi/block out-of-tree fixes for this.

when I try to mount, pm is TRUE (BLK_MQ_REQ_PREEMT set) and that's the
first stack trace I get in this condition, inside of blk_queue_enter():

There is more, but I don't know if that's interesting.

[ 38.642202] CPU: 2 PID: 1522 Comm: mount Not tainted 5.8.0-1-librem5 #487
[ 38.642207] Hardware name: Purism Librem 5r3 (DT)
[ 38.642213] Call trace:
[ 38.642233] dump_backtrace+0x0/0x210
[ 38.642242] show_stack+0x20/0x30
[ 38.642252] dump_stack+0xc8/0x128
[ 38.642262] blk_queue_enter+0x1b8/0x2d8
[ 38.642271] blk_mq_alloc_request+0x54/0xb0
[ 38.642277] blk_get_request+0x34/0x78
[ 38.642286] __scsi_execute+0x60/0x1c8
[ 38.642291] scsi_test_unit_ready+0x88/0x118
[ 38.642298] sd_check_events+0x110/0x158
[ 38.642306] disk_check_events+0x68/0x188
[ 38.642312] disk_clear_events+0x84/0x198
[ 38.642320] check_disk_change+0x38/0x90
[ 38.642325] sd_open+0x60/0x148
[ 38.642330] __blkdev_get+0xcc/0x4c8
[ 38.642335] __blkdev_get+0x278/0x4c8
[ 38.642339] blkdev_get+0x128/0x1a8
[ 38.642345] blkdev_open+0x98/0xb0
[ 38.642354] do_dentry_open+0x130/0x3c8
[ 38.642359] vfs_open+0x34/0x40
[ 38.642366] path_openat+0xa30/0xe40
[ 38.642372] do_filp_open+0x84/0x100
[ 38.642377] do_sys_openat2+0x1f4/0x2b0
[ 38.642382] do_sys_open+0x60/0xa8
(...)

and of course it doesn't work and /dev/sda1 disappears, see the initial
discussion that led to your fix.