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

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