linux-kernel - Re: [PATCH] lightnvm: pblk: adjust the position of the lock

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Date:   Tue, 27 Nov 2018 15:22:01 +0100
From:   Matias Bjørling <mb@...htnvm.io>
To:     javier@...xlabs.com, suhua.tanke@...il.com
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lightnvm: pblk: adjust the position of the lock

On 11/27/2018 01:57 PM, Javier Gonzalez wrote:
> 
>> On 27 Nov 2018, at 02.53, Hua Su <suhua.tanke@...il.com> wrote:
>>
>> Add lock protection for list operations.
>> Signed-off-by: Hua Su <suhua.tanke@...il.com>
>> ---
>> drivers/lightnvm/pblk-core.c | 17 ++++++++++-------
>> 1 file changed, 10 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/lightnvm/pblk-core.c b/drivers/lightnvm/pblk-core.c
>> index 6944aac43b01..e490df217dac 100644
>> --- a/drivers/lightnvm/pblk-core.c
>> +++ b/drivers/lightnvm/pblk-core.c
>> @@ -1286,24 +1286,27 @@ int pblk_line_recov_alloc(struct pblk *pblk, struct pblk_line *line)
>> 	list_del(&line->list);
>>
>> 	ret = pblk_line_prepare(pblk, line);
>> -	if (ret) {
>> -		list_add(&line->list, &l_mg->free_list);
>> -		spin_unlock(&l_mg->free_lock);
>> -		return ret;
>> -	}
>> -	spin_unlock(&l_mg->free_lock);
>> +	if (ret)
>> +		goto out;
>>
>> 	ret = pblk_line_alloc_bitmaps(pblk, line);
>> 	if (ret)
>> -		return ret;
>> +		goto out;
>>
>> 	if (!pblk_line_init_bb(pblk, line, 0)) {
>> 		list_add(&line->list, &l_mg->free_list);
>> +		spin_unlock(&l_mg->free_lock);
>> 		return -EINTR;
>> 	}
>> +	spin_unlock(&l_mg->free_lock);
>>
>> 	pblk_rl_free_lines_dec(&pblk->rl, line, true);
>> 	return 0;
>> +
>> +out:
>> +	list_add(&line->list, &l_mg->free_list);
>> +	spin_unlock(&l_mg->free_lock);
>> +	return ret;
>> }
>>
>> void pblk_line_recov_close(struct pblk *pblk, struct pblk_line *line)
>> --
>> 2.19.1
> 
> This path is only touched by the recovery path, which is single
> threaded, so there is no race condition as is. Also, if recovery fails,
> pblk will not create the instance at all. This said, it would be
> good to protect the list_add on the pblk_line_init_bb() error path in
> case this code is used for some other purpose in the future.

I like your explanation here. Another option is that we could add a 
comment to notify the developer that it safe in this context?

> 
> However, the implementation you propose has some issues due to the
> extended critical zone, which covers pblk_line_alloc_bitmaps(). Here,
> allocations are intentionally non-atomic (i.e., GFP_KERNEL).
> 
> You can instead do the following:
> 
> diff --git i/drivers/lightnvm/pblk-core.c w/drivers/lightnvm/pblk-core.c
> index 615817bf97e3..02c4c1708c45 100644
> --- i/drivers/lightnvm/pblk-core.c
> +++ w/drivers/lightnvm/pblk-core.c
> @@ -1301,15 +1301,22 @@ int pblk_line_recov_alloc(struct pblk *pblk, struct pblk_line *line)
> 
>          ret = pblk_line_alloc_bitmaps(pblk, line);
>          if (ret)
> -               return ret;
> +               goto fail;
> 
>          if (!pblk_line_init_bb(pblk, line, 0)) {
> -               list_add(&line->list, &l_mg->free_list);
> -               return -EINTR;
> +               ret = -EINTR;
> +               goto fail;
>          }
> 
>          pblk_rl_free_lines_dec(&pblk->rl, line, true);
>          return 0;
> +
> +fail:
> +       spin_lock(&l_mg->free_lock);
> +       list_add(&line->list, &l_mg->free_list);
> +       spin_unlock(&l_mg->free_lock);
> +
> +       return ret;
>   }
> 
>   void pblk_line_recov_close(struct pblk *pblk, struct pblk_line *line)
> 
> Javier
> 
>