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

[Richard Weinberger <richard@nod.at>]

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;

Thanks,
//richard