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

Shaun Tancheff
2016-08-04T00:47:36Z

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;

--
Shaun Tancheff