Re: [stable-4.19 8/8] md: Set prev_flush_start and flush_bio in an atomic way

[Jinpu Wang <jinpu.wang@cloud.ionos.com>]

On Thu, Feb 4, 2021 at 4:12 PM Greg KH <gregkh@linuxfoundation.org> wrote:
>
> On Wed, Feb 03, 2021 at 02:20:22PM +0100, Jack Wang wrote:
> > From: Xiao Ni <xni@redhat.com>
> >
> > One customer reports a crash problem which causes by flush request. It
> > triggers a warning before crash.
> >
> >         /* new request after previous flush is completed */
> >         if (ktime_after(req_start, mddev->prev_flush_start)) {
> >                 WARN_ON(mddev->flush_bio);
> >                 mddev->flush_bio = bio;
> >                 bio = NULL;
> >         }
> >
> > The WARN_ON is triggered. We use spin lock to protect prev_flush_start and
> > flush_bio in md_flush_request. But there is no lock protection in
> > md_submit_flush_data. It can set flush_bio to NULL first because of
> > compiler reordering write instructions.
> >
> > For example, flush bio1 sets flush bio to NULL first in
> > md_submit_flush_data. An interrupt or vmware causing an extended stall
> > happen between updating flush_bio and prev_flush_start. Because flush_bio
> > is NULL, flush bio2 can get the lock and submit to underlayer disks. Then
> > flush bio1 updates prev_flush_start after the interrupt or extended stall.
> >
> > Then flush bio3 enters in md_flush_request. The start time req_start is
> > behind prev_flush_start. The flush_bio is not NULL(flush bio2 hasn't
> > finished). So it can trigger the WARN_ON now. Then it calls INIT_WORK
> > again. INIT_WORK() will re-initialize the list pointers in the
> > work_struct, which then can result in a corrupted work list and the
> > work_struct queued a second time. With the work list corrupted, it can
> > lead in invalid work items being used and cause a crash in
> > process_one_work.
> >
> > We need to make sure only one flush bio can be handled at one same time.
> > So add spin lock in md_submit_flush_data to protect prev_flush_start and
> > flush_bio in an atomic way.
> >
> > Reviewed-by: David Jeffery <djeffery@redhat.com>
> > Signed-off-by: Xiao Ni <xni@redhat.com>
> > Signed-off-by: Song Liu <songliubraving@fb.com>
> > [jwang: backport dc5d17a3c39b06aef866afca19245a9cfb533a79 to 4.19]
>
> I can not take patches backported to older kernels that "skip" kernel
> releases.
>
> For example, if I take this into 4.19.y, and then someone moves to 5.4
> or 5.10, they will hit the same issue.
>
> So please provide a backported series for all affected releases, back as
> far as you want, but never skip releases.
>
> I can't take this series, I'll drop it for now and wait for an updated
> set of patches.
>
> thanks,
>
> greg k-h
Hi Greg,

Thanks for reply, only this patch should be backported also to
5.4/5.10, this backport can be applied cleanly to stable/linux-5.4.y
and stable/linux-5.10.y,

I will send the backport for them later today!

Thanks!

J