linux-kernel - Re: [PATCH v3 2/3] scsi: ufs: Fix a race condition btw task management request send and compl

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Date:   Fri, 29 Jan 2021 14:06:18 +0800
From:   Can Guo <cang@...eaurora.org>
To:     Bart Van Assche <bvanassche@....org>
Cc:     jaegeuk@...nel.org, asutoshd@...eaurora.org,
        nguyenb@...eaurora.org, hongwus@...eaurora.org,
        linux-scsi@...r.kernel.org, kernel-team@...roid.com,
        Alim Akhtar <alim.akhtar@...sung.com>,
        Avri Altman <avri.altman@....com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Stanley Chu <stanley.chu@...iatek.com>,
        Bean Huo <beanhuo@...ron.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/3] scsi: ufs: Fix a race condition btw task
 management request send and compl

On 2021-01-29 11:20, Bart Van Assche wrote:
> On 1/27/21 8:16 PM, Can Guo wrote:
>> ufshcd_compl_tm() looks for all 0 bits in the 
>> REG_UTP_TASK_REQ_DOOR_BELL
>> and call complete() for each req who has the req->end_io_data set. 
>> There
>> can be a race condition btw tmc send/compl, because the 
>> req->end_io_data is
>> set, in __ufshcd_issue_tm_cmd(), without host lock protection, so it 
>> is
>> possible that when ufshcd_compl_tm() checks the req->end_io_data, it 
>> is set
>> but the corresponding tag has not been set in 
>> REG_UTP_TASK_REQ_DOOR_BELL.
>> Thus, ufshcd_tmc_handler() may wrongly complete TMRs which have not 
>> been
>> sent out. Fix it by protecting req->end_io_data with host lock, and 
>> let
>> ufshcd_compl_tm() only handle those tm cmds which have been completed
>> instead of looking for 0 bits in the REG_UTP_TASK_REQ_DOOR_BELL.
> 
> I don't know any other block driver that needs locking to protect races
> between submission and completion context. Can the block layer timeout
> mechanism be used instead of the mechanism introduced by this patch,
> e.g. by using blk_execute_rq_nowait() to submit requests? That would
> allow to reuse the existing mechanism in the block layer core to handle
> races between request completion and timeout handling.

This patch is not introducing any new mechanism, it is fixing the
usage of completion (req->end_io_data = c) introduced by commit
69a6c269c097 ("scsi: ufs: Use blk_{get,put}_request() to allocate
and free TMFs"). If you have better idea to get it fixed once for
all, we are glad to take your change to get it fixed asap.

Regards,

Can Guo.

> 
> Thanks,
> 
> Bart.