Re: [PATCH v2 2/4] scsi: ufs: Fix imbalanced scsi_block_reqs_cnt caused by ufshcd_hold()

Can Guo
2020-07-14T04:12:04Z

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@codeaurora.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;
}
...
}