On 08/25/2014 03:25 PM, Richard Weinberger wrote: > 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 <li...@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 <li...@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)
Yes, but the statements: dev->end_sg = 0; dev->request = NULL; at the end of the while loop will only be reached (for FLUSH requests only with my patch applied) when submit_request() succeeds. Otherwise the function do_ubd_request() returns early leaving dev->end_sg and dev->request untouched. > > Thanks, > //richard > -- ___ | | / E-Mail: li...@thorsten-knabe.de |horsten |/\nabe WWW: http://linux.thorsten-knabe.de -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/