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

[Shaun Tancheff]

On Wed, Aug 3, 2016 at 6:47 PM, Mike Christie <mchristi@xxxxxxxxxx> wrote:
> On 08/03/2016 05:33 PM, Ross Zwisler wrote:
>> On Sun, Jun 5, 2016 at 1:32 PM,  <mchristi@xxxxxxxxxx> wrote:
>>> From: Mike Christie <mchristi@xxxxxxxxxx>
>>>
>>> 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@xxxxxxxxxx>
>>> ---
>>>  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@xxxxxxxxxx>
>> 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;

-- 
Shaun Tancheff