Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933313AbcCHLsh (ORCPT ); Tue, 8 Mar 2016 06:48:37 -0500 Received: from smtp.codeaurora.org ([198.145.29.96]:46399 "EHLO smtp.codeaurora.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932243AbcCHLsY (ORCPT ); Tue, 8 Mar 2016 06:48:24 -0500 Message-ID: <8f204a77c853df2c10aeff847f64f1c0.squirrel@us.codeaurora.org> In-Reply-To: <6554458b5f4af5799e68b915626db85f.squirrel@us.codeaurora.org> References: <1456666367-11418-1-git-send-email-ygardi@codeaurora.org> <1456666367-11418-4-git-send-email-ygardi@codeaurora.org> <56D544E6.8040005@suse.de> <2b8282aad0b3edfaf873628edf03513d.squirrel@us.codeaurora.org> <56D7E652.90401@suse.de> <56D833B2.6030104@suse.de> <6554458b5f4af5799e68b915626db85f.squirrel@us.codeaurora.org> Date: Tue, 8 Mar 2016 11:48:22 -0000 Subject: Re: [PATCH v5 03/15] scsi: ufs: implement scsi host timeout handler From: ygardi@codeaurora.org Cc: "Hannes Reinecke" , ygardi@codeaurora.org, james.bottomley@hansenpartnership.com, linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org, linux-arm-msm@vger.kernel.org, santoshsy@gmail.com, linux-scsi-owner@vger.kernel.org, "Gilad Broner" , "Vinayak Holikatti" , "James E.J. Bottomley" , "Martin K. Petersen" , draviv@codeaurora.org User-Agent: SquirrelMail/1.4.22-4.el6 MIME-Version: 1.0 Content-Type: text/plain;charset=iso-8859-1 Content-Transfer-Encoding: 8bit X-Priority: 3 (Normal) Importance: Normal To: unlisted-recipients:; (no To-header on input) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6064 Lines: 174 Hello, Hannes, Re-sending thanks, Yaniv >> On 03/03/2016 05:10 PM, ygardi@codeaurora.org wrote: >>>> On 03/01/2016 09:25 PM, ygardi@codeaurora.org wrote: >>>>>> On 02/28/2016 09:32 PM, Yaniv Gardi wrote: >>>>>>> A race condition exists between request requeueing and scsi layer >>>>>>> error handling: >>>>>>> When UFS driver queuecommand returns a busy status for a request, >>>>>>> it will be requeued and its tag will be freed and set to -1. >>>>>>> At the same time it is possible that the request will timeout and >>>>>>> scsi layer will start error handling for it. The scsi layer reuses >>>>>>> the request and its tag to send error related commands to the >>>>>>> device, >>>>>>> however its tag is no longer valid. >>>>>> Hmm. How can the host return a 'busy' status for a request? >>>>>> From my understanding we have three possibilities: >>>>>> >>>>>> 1) queuecommand returns busy; however, that means that the command >>>>>> has >>>>>> never been send and this issue shouldn't occur >>>>>> 2) The command returns with BUSY status. But in this case it has >>>>>> already >>>>>> been returned, so there cannot be any timeout coming in. >>>>>> 3) The host receives a command with a tag which is already in-use. >>>>>> However, that should have been prevented by the block-layer, which >>>>>> really should ensure that this situation never happens. >>>>>> >>>>>> So either way I look at it, it really looks like a bug and adding a >>>>>> timeout handler will just paper over it. >>>>>> (Not that a timeout handler is a bad idea, in fact I'm convinced >>>>>> that >>>>>> you need one. Just not for this purpose.) >>>>>> >>>>>> So can you elaborate how this 'busy' status comes about? >>>>>> Is the command sent to the device? >>>>>> >>>>>> Cheers, >>>>>> >>>>>> Hannes >>>>> >>>>> >>>>> Hi Hannes, >>>>> >>>>> it's going to be a bit long :) >>>>> I think you are missing the point. >>>>> I will describe a race condition happened to us a while ago, that was >>>>> quite difficult to understand and fix. >>>>> So, this patch is not about the "busy" returning to the scsi dispatch >>>>> routine. it's about the abort triggered after 30 seconds. >>>>> >>>>> imagine a request being queued and sent to the scsi, and then to the >>>>> ufs. >>>>> a timer, initialized to 30 seconds start ticking. >>>>> but the request is never sent to the ufs device, as queuecommand() >>>>> returns >>>>> with "SCSI_MLQUEUE_HOST_BUSY" >>>>> by looking at the code, this could happen, for example: >>>>> err = ufshcd_hold(hba, true); >>>>> if (err) { >>>>> err = SCSI_MLQUEUE_HOST_BUSY; >>>>> goto out; >>>>> } >>>>> >>>> Uuhhh. >>>> You probably should not have pointed me to that piece of code ... >>>> open-coding loops in ufshcd_hold() ... shudder. >>>> (Did I ever review that one? Must've ...) >>>> _Anyway_: sleeping in queuecommand is always a bad idea, as then >>>> precisely those issues you've just described will happen. >>>> >>>> Couldn't you just call >>>> ufshcd_hold(hba, false) >>>> instead of >>>> ufshcd_hold(hba, true) >>>> ? >>>> The request will be requeued more-or-less immediately, avoiding the >>>> issue with timeout handler kicking in. >>>> And the queue will remain blocked until the ungate work item returns, >>>> at >>>> which point I/O submission will continue. >>>> As the request will be requeued to the head of the queue there won't >>>> be >>>> other I/O competing with tags, so it shouldn't have any adverse >>>> effects. >>>> >>>> Wouldn't that work? >>>> >>>> Cheers, >>>> >>>> Hannes >>> >>> Hi Hannes >>> >>> This is a bug, and it should be fixed. >> Oh, definitely agreed. The question is _where_. >> >> >>> if you choose to bypass it, by calling ufshcd_hold(hba, false), not >>> only >>> the race condition is still there, and can pop-out at any other point >>> in >>> the future, but also, not sure what are the consequences of >>> ufshcd_hold(hba, false) unstead of "true". >> Well ... seeing it's your driver, I would've thought _you_ should know >> ... >> >>> so, changing the already tested and working code, (not to return BUSY >>> from >>> queuecommand) is not a fix. >> Hey, I did _not_ suggest not to retury BUSY from queuecommand. >> >> I was suggesting this patch: >> >> diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c >> index 9c1b94b..b9295ad 100644 >> --- a/drivers/scsi/ufs/ufshcd.c >> +++ b/drivers/scsi/ufs/ufshcd.c >> @@ -1388,7 +1388,7 @@ static int ufshcd_queuecommand(struct Scsi_Host >> *host, struct scsi_cmnd *cmd) >> goto out; >> } >> >> - err = ufshcd_hold(hba, true); >> + err = ufshcd_hold(hba, false); >> if (err) { >> err = SCSI_MLQUEUE_HOST_BUSY; >> clear_bit_unlock(tag, &hba->lrb_in_use); >> >> which, by reading the code, should be avoiding this issue. > > > Hannes, > we are not trying to avoid returning BUSY from queuecommand(). > On the contrary. By returning BUSY we actually re-queuing the request > which is exactly what we need to do. > your patch doesn't fix the race condition. > > thanks, > Yaniv > >> I was just asking you if you could give this patch a spin and see if it >> works. If not (for whatever reason) I'm happy to accept your patch. >> But first I would like to have an explanation why the above would _not_ >> work. >> >> Unfortunately I don't have the hardware otherwise I'd be running the >> tests myself. >> >> Cheers, >> >> Hannes >> -- >> Dr. Hannes Reinecke zSeries & Storage >> hare@suse.de +49 911 74053 688 >> SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 N?rnberg >> GF: J. Hawn, J. Guild, F. Imend?rffer, HRB 16746 (AG N?rnberg) >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-scsi" in >> the body of a message to majordomo@vger.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-scsi" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html >