Messages in this thread | | | Date | Mon, 25 Aug 2014 15:25:21 +0200 | From | Richard Weinberger <> | Subject | Re: [PATCH] UML: UBD: Fix for processes stuck in D state forever in UserModeLinux |
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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
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