On 2022-08-25 16:19, Logan Gunthorpe wrote:

Given the conditions of hitting the bug, I fully expected this to be an
issue in the raid code, but unless I'm missing something, it sure looks
to me like a deadlock issue in the wbt code, which makes me wonder why
nobody else has hit it. Is there something else I'm missing that are
supposed to be waking up these processes? Or something weird about the
raid5+journal+loop code that is causing wb_recent_wait() to always be false?

I've made some progress on this nasty bug. I've got far enough to know it's not
related to the blk-wbt or the block layer.

Turns out a bunch of bios are stuck queued in a blk_plug in the md_raid5 
thread while that thread appears to be stuck in an infinite loop (so it never
schedules or does anything to flush the plug). 

I'm still debugging to try and find out the root cause of that infinite loop, 
but I just wanted to send an update that the previous place I was stuck at
was not correct.

Logan