linux-kernel - Re: [PATCH 1/4] scsi: ufs: atomic update for clkgating

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Date:   Tue, 20 Oct 2020 10:30:16 +0800
From:   Can Guo <cang@...eaurora.org>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
        kernel-team@...roid.com, Jaegeuk Kim <jaegeuk@...gle.com>,
        Alim Akhtar <alim.akhtar@...sung.com>,
        Avri Altman <avri.altman@....com>
Subject: Re: [PATCH 1/4] scsi: ufs: atomic update for clkgating_enable

On 2020-10-06 06:36, Jaegeuk Kim wrote:
> From: Jaegeuk Kim <jaegeuk@...gle.com>
> 
> When giving a stress test which enables/disables clkgating, we hit 
> device
> timeout sometimes. This patch avoids subtle racy condition to address 
> it.
> 
> Cc: Alim Akhtar <alim.akhtar@...sung.com>
> Cc: Avri Altman <avri.altman@....com>
> Cc: Can Guo <cang@...eaurora.org>
> Signed-off-by: Jaegeuk Kim <jaegeuk@...gle.com>
> ---
>  drivers/scsi/ufs/ufshcd.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
> index 1d157ff58d817..d929c3d1e58cc 100644
> --- a/drivers/scsi/ufs/ufshcd.c
> +++ b/drivers/scsi/ufs/ufshcd.c
> @@ -1791,19 +1791,19 @@ static ssize_t
> ufshcd_clkgate_enable_store(struct device *dev,
>  		return -EINVAL;
> 
>  	value = !!value;
> +
> +	spin_lock_irqsave(hba->host->host_lock, flags);
>  	if (value == hba->clk_gating.is_enabled)
>  		goto out;
> 
> -	if (value) {
> -		ufshcd_release(hba);
> -	} else {
> -		spin_lock_irqsave(hba->host->host_lock, flags);
> +	if (value)
> +		hba->clk_gating.active_reqs--;
> +	else
>  		hba->clk_gating.active_reqs++;
> -		spin_unlock_irqrestore(hba->host->host_lock, flags);
> -	}
> 
>  	hba->clk_gating.is_enabled = value;
>  out:
> +	spin_unlock_irqrestore(hba->host->host_lock, flags);
>  	return count;
>  }

I agree that we should protect the flag "is_enabled" with spin lock,
but I prefer the old logic of calling ufshcd_release() instead of
just doing hba->clk_gating.active_reqs--, you can use 
__ufshcd_release(),
which is free of locking.

Thanks,

Can Guo.