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Date: Fri, 05 Dec 2014 22:08:19 +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()
Tanya,
Am 05.12.2014 um 17:54 schrieb Tanya Brokhman:
> Hi Richard
>
> On 12/5/2014 3:20 PM, Richard Weinberger wrote:
>> Tanya,
>>
>> Am 05.12.2014 um 14:09 schrieb Tanya Brokhman:
>>> On 11/24/2014 3:20 PM, Richard Weinberger wrote:
>>>> ubi_wl_get_peb() has two problems, it reads the pool
>>>> size and usage counters without any protection.
>>>> While reading one value would be perfectly fine it reads multiple
>>>> values and compares them. This is racy and can lead to incorrect
>>>> pool handling.
>>>> Furthermore ubi_update_fastmap() is called without wl_lock held,
>>>> before incrementing the used counter it needs to be checked again.
>>>
>>> I didn't see where you fixed the ubi_update_fastmap issue you just mentioned.
>>
>> This is exactly what you're questioning below.
>> We have to recheck as the pool counter could have changed.
>>
>
> Oh, I understood the commit msg a bit differently, but now I see that it was my mistake. thanks!
>
>>>> It could happen that another thread consumed all PEBs from the
>>>> pool and the counter goes beyond ->size.
>>>>
>>>> Signed-off-by: Richard Weinberger <richard@....at>
>>>> ---
>>>> drivers/mtd/ubi/ubi.h | 3 ++-
>>>> drivers/mtd/ubi/wl.c | 34 +++++++++++++++++++++++-----------
>>>> 2 files changed, 25 insertions(+), 12 deletions(-)
>>>>
>>>> diff --git a/drivers/mtd/ubi/ubi.h b/drivers/mtd/ubi/ubi.h
>>>> index 04c4c05..d672412 100644
>>>> --- a/drivers/mtd/ubi/ubi.h
>>>> +++ b/drivers/mtd/ubi/ubi.h
>>>> @@ -439,7 +439,8 @@ struct ubi_debug_info {
>>>> * @pq_head: protection queue head
>>>> * @wl_lock: protects the @used, @free, @pq, @pq_head, @lookuptbl, @move_from,
>>>> * @move_to, @move_to_put @erase_pending, @wl_scheduled, @works,
>>>> - * @erroneous, @erroneous_peb_count, and @fm_work_scheduled fields
>>>> + * @erroneous, @erroneous_peb_count, @fm_work_scheduled, @fm_pool,
>>>> + * and @fm_wl_pool fields
>>>> * @move_mutex: serializes eraseblock moves
>>>> * @work_sem: used to wait for all the scheduled works to finish and prevent
>>>> * new works from being submitted
>>>> diff --git a/drivers/mtd/ubi/wl.c b/drivers/mtd/ubi/wl.c
>>>> index cb2e571..7730b97 100644
>>>> --- a/drivers/mtd/ubi/wl.c
>>>> +++ b/drivers/mtd/ubi/wl.c
>>>> @@ -629,24 +629,36 @@ void ubi_refill_pools(struct ubi_device *ubi)
>>>> */
>>>> int ubi_wl_get_peb(struct ubi_device *ubi)
>>>> {
>>>> - int ret;
>>>> + int ret, retried = 0;
>>>> struct ubi_fm_pool *pool = &ubi->fm_pool;
>>>> struct ubi_fm_pool *wl_pool = &ubi->fm_wl_pool;
>>>>
>>>> - if (!pool->size || !wl_pool->size || pool->used == pool->size ||
>>>> - wl_pool->used == wl_pool->size)
>>>> +again:
>>>> + spin_lock(&ubi->wl_lock);
>>>> + /* We check here also for the WL pool because at this point we can
>>>> + * refill the WL pool synchronous. */
>>>> + if (pool->used == pool->size || wl_pool->used == wl_pool->size) {
>>>> + spin_unlock(&ubi->wl_lock);
>>>> ubi_update_fastmap(ubi);
>>>> -
>>>> - /* we got not a single free PEB */
>>>> - if (!pool->size)
>>>> - ret = -ENOSPC;
>>>> - else {
>>>> spin_lock(&ubi->wl_lock);
>>>> - ret = pool->pebs[pool->used++];
>>>> - prot_queue_add(ubi, ubi->lookuptbl[ret]);
>>>> + }
>>>> +
>>>> + if (pool->used == pool->size) {
>>>
>>> Im confused about this "if" condition. You just tested pool->used == pool->size in the previous "if". If in the previous if pool->used != pool->size and wl_pool->used !=
>>> wl_pool->size, you didn't enter, the lock is still held so pool->used != pool->size still. If in the previos "if" wl_pool->used == wl_pool->size was true nd tou released the lock,
>>> ubi_update_fastmap(ubi) was called, which refills the pools. So again, if pools were refilled pool->used would be 0 here and pool->size > 0.
>>>
>>> So in both cases I don't see how at this point pool->used == pool->size could ever be true?
>>
>> If we enter the "if (pool->used == pool->size || wl_pool->used == wl_pool->size) {" branch we unlock wl_lock and call ubi_update_fastmap().
>> Another thread can enter ubi_wl_get_peb() and alter the pool counter. So we have to recheck the counter after taking wl_lock again.
>
> hmmm... ok. Perhaps a comment could be added in the code to explain this case in a few words?
>
Makes sense, I'll add a comment.
>>>> 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().
Thanks,
//richard
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