[PATCH 1/1] Set prev_flush_start and flush_bio in an atomic way

[PATCH 1/1] Set prev_flush_start and flush_bio in an atomic way

[Xiao Ni]

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
exteneded 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>
---
 drivers/md/md.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index c42af46..2746d5c 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -639,8 +639,10 @@ static void md_submit_flush_data(struct work_struct *ws)
 	 * could wait for this and below md_handle_request could wait for those
 	 * bios because of suspend check
 	 */
+	spin_lock_irq(&mddev->lock);
 	mddev->prev_flush_start = mddev->start_flush;
 	mddev->flush_bio = NULL;
+	spin_unlock_irq(&mddev->lock);
 	wake_up(&mddev->sb_wait);
 
 	if (bio->bi_iter.bi_size == 0) {
-- 
2.7.5

Re: [PATCH 1/1] Set prev_flush_start and flush_bio in an atomic way

[Xiao Ni]

Hi Song

Could you review this patch

Regards
Xiao

On 12/10/2020 02:33 PM, Xiao Ni wrote:
> 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
> exteneded 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>
> ---
>   drivers/md/md.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index c42af46..2746d5c 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -639,8 +639,10 @@ static void md_submit_flush_data(struct work_struct *ws)
>   	 * could wait for this and below md_handle_request could wait for those
>   	 * bios because of suspend check
>   	 */
> +	spin_lock_irq(&mddev->lock);
>   	mddev->prev_flush_start = mddev->start_flush;
>   	mddev->flush_bio = NULL;
> +	spin_unlock_irq(&mddev->lock);
>   	wake_up(&mddev->sb_wait);
>   
>   	if (bio->bi_iter.bi_size == 0) {

Re: [PATCH 1/1] Set prev_flush_start and flush_bio in an atomic way

[Song Liu]

> On Jan 12, 2021, at 1:24 AM, Xiao Ni <xni@redhat.com> wrote:
> 
> Hi Song
> 
> Could you review this patch
> 

I am sorry for the delay. Just applied it to md-fixes. 

I made some changes to it
 1. add md: prefix to the commit log subject;
 2. Adjust the width of the commit log to 75 characters. 

For future tests please run ./scripts/checkpatch.pl on it. 

Thanks,
Song

> Regards
> Xiao
> 
> On 12/10/2020 02:33 PM, Xiao Ni wrote:
>> 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
>> exteneded 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>
>> ---
>>  drivers/md/md.c | 2 ++
>>  1 file changed, 2 insertions(+)
>> 
>> diff --git a/drivers/md/md.c b/drivers/md/md.c
>> index c42af46..2746d5c 100644
>> --- a/drivers/md/md.c
>> +++ b/drivers/md/md.c
>> @@ -639,8 +639,10 @@ static void md_submit_flush_data(struct work_struct *ws)
>>  	 * could wait for this and below md_handle_request could wait for those
>>  	 * bios because of suspend check
>>  	 */
>> +	spin_lock_irq(&mddev->lock);
>>  	mddev->prev_flush_start = mddev->start_flush;
>>  	mddev->flush_bio = NULL;
>> +	spin_unlock_irq(&mddev->lock);
>>  	wake_up(&mddev->sb_wait);
>>    	if (bio->bi_iter.bi_size == 0) {
>