Re: [PATCH v4 4/4] blk-flush: reuse rq queuelist in flush state machine

[Christoph Hellwig]

On Wed, May 29, 2024 at 04:50:02PM +0800, Chengming Zhou wrote:
> Yes, because we use list_move_tail() in the flush sequences. Maybe we can
> just use list_add_tail() so we don't need the queuelist initialized. It
> should be ok since rq can't be on any list when PREFLUSH or POSTFLUSH,
> so there isn't any move actually.

Sounds good.

> But now I'm concerned that rq->queuelist maybe changed by driver after
> request end?

How could the driver change it?

> > Also, just out of interest: Can you estimate whether this issue is
> > specific to software RAID setups, or could similar NULL pointer
> > dereferences also happen in setups without software RAID?
> 
> I think it can also happen without software RAID.

Seems to be about batch allocation.  So you either need a plug in
the stacking device, or io_uring.  I guess people aren't using the
io_uring high performance options on devices with a write cache
all that much, as that should immediately reproduce the problem.