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From: Bart Van Assche <bvanassche@acm.org>
To: Can Guo <cang@codeaurora.org>
Cc: "Martin K . Petersen" <martin.petersen@oracle.com>,
	"James E . J . Bottomley" <jejb@linux.vnet.ibm.com>,
	linux-scsi@vger.kernel.org, Bean Huo <beanhuo@micron.com>,
	Avri Altman <avri.altman@wdc.com>,
	Stanley Chu <stanley.chu@mediatek.com>,
	Tomas Winkler <tomas.winkler@intel.com>
Subject: Re: [PATCH 6/6] ufs: Remove the SCSI timeout handler
Date: Thu, 28 May 2020 09:12:05 -0700	[thread overview]
Message-ID: <1728e2d6-5b00-e71f-5476-b082f4201aa1@acm.org> (raw)
In-Reply-To: <4fe9074323178a0b006f08402dd08b51@codeaurora.org>

On 2020-05-28 02:47, Can Guo wrote:
> Hi Bart,
> 
> On 2019-12-25 06:02, Bart Van Assche wrote:
>> The UFS SCSI timeout handler was needed to compensate that
>> ufshcd_queuecommand() could return SCSI_MLQUEUE_HOST_BUSY for a long
>> time. Commit a276c19e3e98 ("scsi: ufs: Avoid busy-waiting by eliminating
>> tag conflicts") fixed this so the timeout handler is no longer necessary.
>>
>> See also commit f550c65b543b ("scsi: ufs: implement scsi host timeout
>> handler").
>>
> 
> Sorry for bugging you on this old change. I am afraid we may need to add
> this timeout handler back. Because there is till chances that a request
> gets stuck somewhere in ufshcd_queuecommand() path before
> ufshcd_send_command() gets called. e.g.
> 
> ufshcd_queuecommand()
> ->ufshcd_map_sg()
> -->scsi_dma_map()
> --->dma_map_sg()
> ---->dev->ops->map_sg()
> 
> map_sg() ops may get stuck. map_sg() method can vary on different platforms
> based on actual IOMMU engines. We cannot gaurantee map_sg() ops must return
> immediately as we don't know what is actually inside map_sg() ops.
> 
> And if it gets stuck there for a long time till the request times out,
> without
> the UFS timeout handler, scsi layer will try to abort this request from UFS
> driver by calling ufshcd_abort() eventually. ufshcd_abort() will think this
> request has been completed due to its tag is not in hba->outstanding_reqs
> or UFS host's door bell reg. However, actually, this request is still in
> ufshcd_queuecommand() path. I don't need to continue on the subsequent
> impact
> to UFS driver if ufshcd_abort() happens in this case. This is a corner
> case,
> but it is still possible (I did see map_sg() ops hangs on real devices).
> 
> Having the UFS timeout handler back will prevent this situation as UFS
> timeout
> handler checks if the tag is in hba->outstanding_reqs (for our case, it
> is not
> in there), if no, it returns BLK_EH_RESET_TIMER so that scsi/block layer
> will
> keep waiting.
> 
> What do you think? Please let me know your ideas on this, thanks!

Hi Can,

I see the following issues with the above proposal:
- Although I haven't been able to find explicit documentation of this, I
  think that dma_map_sg() must not sleep. If it would sleep that would
  break most block and SCSI drivers because many of these drivers call
  dma_map_sg() from their .queue_rq() or .queuecommand() implementation
  and if BLK_MQ_F_BLOCKING has not been set these functions must not
  sleep.
- A timeout handler must not be invoked while .queuecommand() is still
  in progress. The SCSI core calls blk_mq_start_request() before it
  calls ufshcd_queuecommand(). The blk_mq_start_request() activates the
  block layer timeout mechanism. ufshcd_queuecommand() must have
  finished before the block layer timeout handler is activated.

Please fix the root cause, namely the map_sg implementation that may get
stuck.

Thanks,

Bart.

  reply	other threads:[~2020-05-28 16:12 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-24 22:02 [PATCH 0/6] Six UFS patches Bart Van Assche
2019-12-24 22:02 ` [PATCH 1/6] ufs: Fix indentation in ufshcd_query_attr_retry() Bart Van Assche
2019-12-25  7:16   ` Stanley Chu
2019-12-25  8:17   ` Can Guo
2019-12-27  1:13   ` Alim Akhtar
2019-12-24 22:02 ` [PATCH 2/6] ufs: Make ufshcd_add_command_trace() easier to read Bart Van Assche
2019-12-25  7:16   ` Stanley Chu
2019-12-25  8:17   ` Can Guo
2019-12-27  1:17   ` Alim Akhtar
2019-12-24 22:02 ` [PATCH 3/6] ufs: Make ufshcd_prepare_utp_scsi_cmd_upiu() " Bart Van Assche
2019-12-25  7:17   ` Stanley Chu
2019-12-25  8:18   ` Can Guo
2019-12-24 22:02 ` [PATCH 4/6] ufs: Fix a race condition in the tracing code Bart Van Assche
2019-12-25 10:59   ` Can Guo
2019-12-26 17:35     ` Bart Van Assche
2019-12-27  5:50       ` Can Guo
2019-12-27  1:21   ` Alim Akhtar
2019-12-24 22:02 ` [PATCH 5/6] ufs: Remove superfluous memory barriers Bart Van Assche
2019-12-25 10:31   ` Can Guo
2019-12-26 17:36     ` Bart Van Assche
2019-12-24 22:02 ` [PATCH 6/6] ufs: Remove the SCSI timeout handler Bart Van Assche
2019-12-25 11:02   ` Can Guo
2019-12-27  1:24   ` Alim Akhtar
2020-05-28  9:47   ` Can Guo
2020-05-28 16:12     ` Bart Van Assche [this message]
2020-05-29  1:39       ` Can Guo
2020-05-29  3:56         ` Bart Van Assche
2020-01-03  2:58 ` [PATCH 0/6] Six UFS patches Martin K. Petersen

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