Re: [PATCH] UML: UBD: Fix for processes stuck in D state forever in UserModeLinux

[Richard Weinberger <richard@nod.at>]

Am 25.08.2014 01:02, schrieb Thorsten Knabe:
> On 08/24/2014 02:11 PM, Richard Weinberger wrote:
>> Am 23.08.2014 19:43, schrieb Thorsten Knabe:
>>> Hi Richard.
>>>
>>> On 08/23/2014 05:34 PM, Richard Weinberger wrote:
>>>> Hi!
>>>>
>>>> Am 23.08.2014 15:47, schrieb Thorsten Knabe:
>>>>> From: Thorsten Knabe <linux@thorsten-knabe.de>
>>>>>
>>>>> UML: UBD: Fix for processes stuck in D state forever in UserModeLinux.
>>>>>
>>>>> Starting with Linux 3.12 processes get stuck in D state forever in
>>>>> UserModeLinux under sync heavy workloads. This bug was introduced by
>>>>> commit 805f11a0d5 (um: ubd: Add REQ_FLUSH suppport).
>>>>> Fix bug by adding a check if FLUSH request was successfully submitted to
>>>>> the I/O thread and keeping the FLUSH request on the request queue on
>>>>> submission failures.
>>>>>
>>>>> Fixes: 805f11a0d5 (um: ubd: Add REQ_FLUSH suppport)
>>>>> Signed-off-by: Thorsten Knabe <linux@thorsten-knabe.de>
>>>>
>>>> Thanks a lot for hunting this issue down.
>>>>
>>>>> ---
>>>>> Patch applies to 3.16.1.
>>>>>
>>>>> diff --git a/arch/um/drivers/ubd_kern.c b/arch/um/drivers/ubd_kern.c
>>>>> index 3716e69..b7d2840 100644
>>>>> --- a/arch/um/drivers/ubd_kern.c
>>>>> +++ b/arch/um/drivers/ubd_kern.c
>>>>> @@ -1277,7 +1277,7 @@ static void do_ubd_request(struct request_queue *q)
>>>>>
>>>>>  	while(1){
>>>>>  		struct ubd *dev = q->queuedata;
>>>>> -		if(dev->end_sg == 0){
>>>>> +		if(dev->request == NULL){
>>>>
>>>> Why do we need this specific change?
>>>
>>> This change is required, because for FLUSH requests dev->end_sg is
>>> initialized to 0 by blk_rq_map_sg() a few lines above, as FLUSH requests
>>> have no data blocks attached to themselves.
>>
>> You meant "below"? Looks like I really miss something here.
>> At the bottom of the while(1) loop we have
>>                 dev->end_sg = 0;
>>                 dev->request = NULL;
> 
> No. The problematic line is:
>                         dev->end_sg = blk_rq_map_sg(q, req, dev->sg);
> and blk_rq_map_sg() returning 0 for REQ_FLUSH requests, because they
> have no associated data blocks.
> 
> Hence on the next iteration of the while(1) loop:
>                 if(dev->end_sg == 0){

At the bottom of the while loop dev->end_sg will be set to 0 anyway, this is what puzzles
me so hard.
(And dev->request too)

Thanks,
//richard