Re: [PATCH 37/45] drivers: use req op accessor

[Mike Christie <mchristi@redhat.com>]

On 08/03/2016 07:30 PM, Shaun Tancheff wrote:
> On Wed, Aug 3, 2016 at 6:47 PM, Mike Christie <mchristi@redhat.com> wrote:
>> On 08/03/2016 05:33 PM, Ross Zwisler wrote:
>>> On Sun, Jun 5, 2016 at 1:32 PM,  <mchristi@redhat.com> wrote:
>>>> From: Mike Christie <mchristi@redhat.com>
>>>>
>>>> The req operation REQ_OP is separated from the rq_flag_bits
>>>> definition. This converts the block layer drivers to
>>>> use req_op to get the op from the request struct.
>>>>
>>>> Signed-off-by: Mike Christie <mchristi@redhat.com>
>>>> ---
>>>>  drivers/block/loop.c              |  6 +++---
>>>>  drivers/block/mtip32xx/mtip32xx.c |  2 +-
>>>>  drivers/block/nbd.c               |  2 +-
>>>>  drivers/block/rbd.c               |  4 ++--
>>>>  drivers/block/xen-blkfront.c      |  8 +++++---
>>>>  drivers/ide/ide-floppy.c          |  2 +-
>>>>  drivers/md/dm.c                   |  2 +-
>>>>  drivers/mmc/card/block.c          |  7 +++----
>>>>  drivers/mmc/card/queue.c          |  6 ++----
>>>
>>> Dave Chinner reported a deadlock with XFS + DAX, which I reproduced
>>> and bisected to this commit:
>>>
>>> commit c2df40dfb8c015211ec55f4b1dd0587f875c7b34
>>> Author: Mike Christie <mchristi@redhat.com>
>>> Date:   Sun Jun 5 14:32:17 2016 -0500
>>> drivers: use req op accessor
>>>
>>> Here are the steps to reproduce the deadlock with a BRD ramdisk:
>>>
>>> mkfs.xfs -f /dev/ram0
>>> mount -o dax /dev/ram0 /mnt/scratch
>>
>> When using ramdisks, we need the attached patch like in your other bug
>> report. I think it will fix some hangs people are seeing.
>>
>> I do not think that it should cause the failure to run issue you saw
>> when doing generic/008 and ext2.
>>
> 
> I think the translation in loop.c is suspicious here:
> 
>     "if use DIO && not (a flush_flag or discard_flag)"
> should translate to:
>     "if use DIO && not ((a flush_flag) || op == discard)"
> 
> But in the patch I read:
>     "if use DIO && ((not a flush_flag) || op == discard)
> 
> Which would have DIO && discards follow the AIO path?
> 
> So I would humbly suggest something like the following
>     (on top of commit c2df40dfb8c015211ec55f4b1dd0587f875c7b34):
> [Please excuse the messed up patch format ... gmail eats tabs]
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index b9b737c..0754d83 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -1659,8 +1659,9 @@ static int loop_queue_rq(struct blk_mq_hw_ctx *hctx,
>         if (lo->lo_state != Lo_bound)
>                 return -EIO;
> 
> -       if (lo->use_dio && (!(cmd->rq->cmd_flags & REQ_FLUSH) ||
> -           req_op(cmd->rq) == REQ_OP_DISCARD))
> +       if (lo->use_dio && !(
> +           (cmd->rq->cmd_flags & REQ_FLUSH) ||
> +            req_op(cmd->rq) == REQ_OP_DISCARD))
>                 cmd->use_aio = true;
>         else
>                 cmd->use_aio = false;
> 

You are right. The translation was bad and your code above is correct.

I think we need my patch in the other mail though too, because for the
rw_page user case if WB_SYNC_ALL is set, then the IO gets sent down as a
read instead of a write.