linux-kernel - Re: [PATCH 1/2] staging: zram: minimize `slot_free

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Date:	Mon, 09 Sep 2013 18:10:07 +0200
From:	Jerome Marchand <jmarchan@...hat.com>
To:	Dan Carpenter <dan.carpenter@...cle.com>
CC:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	devel@...verdev.osuosl.org, Minchan Kim <minchan@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] staging: zram: minimize `slot_free_lock' usage (v2)

On 09/09/2013 03:46 PM, Jerome Marchand wrote:
> On 09/09/2013 03:21 PM, Dan Carpenter wrote:
>> On Mon, Sep 09, 2013 at 03:49:42PM +0300, Sergey Senozhatsky wrote:
>>>>> Calling handle_pending_slot_free() for every RW operation may
>>>>> cause unneccessary slot_free_lock locking, because most likely
>>>>> process will see NULL slot_free_rq. handle_pending_slot_free()
>>>>> only when current detects that slot_free_rq is not NULL.
>>>>>
>>>>> v2: protect handle_pending_slot_free() with zram rw_lock.
>>>>>
>>>>
>>>> zram->slot_free_lock protects zram->slot_free_rq but shouldn't the zram
>>>> rw_lock be wrapped around the whole operation like the original code
>>>> does?  I don't know the zram code, but the original looks like it makes
>>>> sense but in this one it looks like the locks are duplicative.
>>>>
>>>> Is the down_read() in the original code be changed to down_write()?
>>>>
>>>
>>> I'm not touching locking around existing READ/WRITE commands.
>>>
>>
>> Your patch does change the locking because now instead of taking the
>> zram lock once it takes it and then drops it and then retakes it.  This
>> looks potentially racy to me but I don't know the code so I will defer
>> to any zram maintainer.
> 
> You're right. Nothing prevents zram_slot_free_notify() to repopulate the
> free slot queue while we drop the lock.
> 
> Actually, the original code is already racy. handle_pending_slot_free()
> modifies zram->table while holding only a read lock. It needs to hold a
> write lock to do that. Using down_write for all requests would obviously
> fix that, but at the cost of read performance.

Now I think we can drop the call to handle_pending_slot_free() in
zram_bvec_rw() altogether. As long as the write lock is held when
handle_pending_slot_free() is called, there is no race. It's no different
from any write request and the current code handles R/W concurrency
already.

Jerome

> 
>>
>> 1) You haven't given us any performance numbers so it's not clear if the
>>    locking is even a problem.
>>
>> 2) The v2 patch introduces an obvious deadlock in zram_slot_free()
>>    because now we take the rw_lock twice.  Fix your testing to catch
>>    this kind of bug next time.
>>
>> 3) Explain why it is safe to test zram->slot_free_rq when we are not
>>    holding the lock.  I think it is unsafe.  I don't want to even think
>>    about it without the numbers.
>>
>> regards,
>> dan carpenter
>>
> 
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