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

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Message-ID: <548421AB.90303@nod.at>
Date:	Sun, 07 Dec 2014 10:45:15 +0100
From:	Richard Weinberger <richard@....at>
To:	Tanya Brokhman <tlinder@...eaurora.org>, 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()

Am 07.12.2014 um 08:36 schrieb Tanya Brokhman:
> 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.

If there is room for improvement I'm all open for an extra patch set all over UBI. :-)

Thanks,
//richard
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