linux-kernel - Re: [PATCH v2 2/4] scsi: ufs: Fix imbalanced scsi_block_reqs_cnt caused by ufshcd

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Date:   Tue, 14 Jul 2020 12:11:01 +0800
From:   Can Guo <cang@...eaurora.org>
To:     Bart Van Assche <bvanassche@....org>
Cc:     asutoshd@...eaurora.org, nguyenb@...eaurora.org,
        hongwus@...eaurora.org, rnayak@...eaurora.org,
        linux-scsi@...r.kernel.org, kernel-team@...roid.com,
        saravanak@...gle.com, salyzyn@...gle.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 v2 2/4] scsi: ufs: Fix imbalanced scsi_block_reqs_cnt
 caused by ufshcd_hold()

On 2020-07-14 11:41, Bart Van Assche wrote:
> On 2020-07-13 19:28, Can Guo wrote:
>> The scsi_block_reqs_cnt increased in ufshcd_hold() is supposed to be
>> decreased back in ufshcd_ungate_work() in a paired way. However, if
>> specific ufshcd_hold/release sequences are met, it is possible that
>> scsi_block_reqs_cnt is increased twice but only one ungate work is
>> queued. To make sure scsi_block_reqs_cnt is handled by ufshcd_hold() 
>> and
>> ufshcd_ungate_work() in a paired way, increase it only if queue_work()
>> returns true.
>> 
>> Signed-off-by: Can Guo <cang@...eaurora.org>
>> ---
>>  drivers/scsi/ufs/ufshcd.c | 6 +++---
>>  1 file changed, 3 insertions(+), 3 deletions(-)
>> 
>> diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
>> index ebf7a95..33214bb 100644
>> --- a/drivers/scsi/ufs/ufshcd.c
>> +++ b/drivers/scsi/ufs/ufshcd.c
>> @@ -1611,12 +1611,12 @@ int ufshcd_hold(struct ufs_hba *hba, bool 
>> async)
>>  		 */
>>  		/* fallthrough */
>>  	case CLKS_OFF:
>> -		ufshcd_scsi_block_requests(hba);
>>  		hba->clk_gating.state = REQ_CLKS_ON;
>>  		trace_ufshcd_clk_gating(dev_name(hba->dev),
>>  					hba->clk_gating.state);
>> -		queue_work(hba->clk_gating.clk_gating_workq,
>> -			   &hba->clk_gating.ungate_work);
>> +		if (queue_work(hba->clk_gating.clk_gating_workq,
>> +			       &hba->clk_gating.ungate_work))
>> +			ufshcd_scsi_block_requests(hba);
>>  		/*
>>  		 * fall through to check if we should wait for this
>>  		 * work to be done or not.
> 
> Since "ungate_work" involves calling ufshcd_scsi_unblock_requests() and
> since this patch changes the order in which 
> ufshcd_scsi_block_requests()
> and queue_work() are called, I think this patch introduces a race
> condition. Has it been considered to leave the 
> ufshcd_scsi_block_requests()
> call where it is and to call ufshcd_scsi_unblock_requests() if
> queue_work() fails?
> 
> Thanks,
> 
> Bart.

Hi Bart,

The racing does not exist due to we still hold the spin lock here. 
Before
release the spin lock, the ungate_work, even it starts to run, cannot
move forward as it needs to require the spin lock once in the entrance.

static void ufshcd_ungate_work(struct work_struct *work)
{
...
         spin_lock_irqsave(hba->host->host_lock, flags);
         if (hba->clk_gating.state == CLKS_ON) {
                 spin_unlock_irqrestore(hba->host->host_lock, flags);
                 goto unblock_reqs;
         }
...
}