linux-kernel - Re: [PATCH 4/6] UBI: Fastmap: Fix races in ubi_wl_get

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Date:	Sun, 07 Dec 2014 09:36:37 +0200
From:	Tanya Brokhman <tlinder@...eaurora.org>
To:	Richard Weinberger <richard@....at>, dedekind1@...il.com
CC:	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] UBI: Fastmap: Fix races in ubi_wl_get_peb()

On 12/5/2014 11:08 PM, Richard Weinberger wrote:

>
>>>>>             spin_unlock(&ubi->wl_lock);
>>>>> +        if (retried) {
>>>>> +            ubi_err(ubi, "Unable to get a free PEB from user WL pool");
>>>>> +            ret = -ENOSPC;
>>>>> +            goto out;
>>>>> +        }
>>>>> +        retried = 1;
>>>>
>>>> Why did you decide to retry in this function? and why only 1 retry attempt? I'm not against it, trying to understand the logic.
>>>
>>> Because failing immediately with -ENOSPC is not nice.
>>
>> Why not? this is what was done before....
>
> The behavior from before was not good.
> If we return here a -ENOSPC it is not because we ran out of free PEBs, it is because the pool contains
> no free PEBs and needs refilling.
> As between refilling the pool and requesting a fresh PEB from it another thread could "steal" all PEBs
> we retry.
>
>> I think what I really bothers me in this case is that you don't sleep, you branch immediately to retry again, so the chances that there will be context switch and free pebs appear
>> aren't that high.
>> I'm used to functions using some sort of "retry" logic to sleep before retrying. Of course sleeping isn't a good idea here. That's why the "retry" bugs me a bit.
>
> You mean a cond_resched()?
> This retry-logic is common pattern in UBI. For exmaple see ubi_wl_put_peb().

you're right. didn't pay much attention to ubi_wl_put_peb() before. 
don't like it there either :)
perhaps we can rethink this later for both cases.

>
> Thanks,
> //richard
>


Thanks,
Tanya Brokhman
-- 
Qualcomm Israel, on behalf of Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora 
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