linux-kernel - Re: [PATCH 2/3] ocfs2: Remove a useless spinlock

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Date:   Wed, 20 Jul 2022 17:48:55 +0800
From:   Joseph Qi <joseph.qi@...ux.alibaba.com>
To:     Marion & Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     David.Laight@...LAB.COM, jlbec@...lplan.org,
        kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org,
        mark@...heh.com, ocfs2-devel@....oracle.com
Subject: Re: [PATCH 2/3] ocfs2: Remove a useless spinlock



On 7/20/22 4:26 PM, Marion & Christophe JAILLET wrote:
> 
> Le 20/07/2022 à 03:59, Joseph Qi a écrit :
>>
>> On 7/19/22 9:25 PM, Christophe JAILLET wrote:
>>> Le 19/07/2022 à 12:24, David Laight a écrit :
>>>> From: Christophe JAILLET
>>>>> Sent: 19 July 2022 11:02
>>>>>
>>>>> 'node_map_lock' is a spinlock only used to protect calls to set_bit(),
>>>>> clear_bit() and test_bit().
>>>>>
>>>>> {set|clear}_bit() are already atomic and don't need this extra spinlock.
>>>>> test_bit() only reads the bitmap for a given bit.
>>>>>
>>>>> Remove this useless spinlock.
>>>> It looks to me like the calling code is racy
>>>> unless there is another lock in the callers.
>>> The call chains are:
>>>    ocfs2_recover_orphans()
>>>      ocfs2_mark_recovering_orphan_dir()
>>>        spin_lock(&osb->osb_lock);        <-- osb_lock spinlock
>>>        ocfs2_node_map_set_bit()            <-- uses node_map_lock
>>>        ...
>>>        spin_unlock(&osb->osb_lock);
>>>      ...
>>>      ocfs2_clear_recovering_orphan_dir()
>>>        ocfs2_node_map_clear_bit()        <-- uses node_map_lock
>>>                              osb_lock is NOT taken
>>>
>>>
>>>    ocfs2_check_orphan_recovery_state()
>>>      spin_lock(&osb->osb_lock);            <-- osb_lock spinlock
>>>      ...
>>>      ocfs2_node_map_test_bit()            <-- uses node_map_lock
>>>      ...
>>>      spin_unlock(&osb->osb_lock);
>>>
>>>
>>> So the code looks already protected by the 'osb_lock' spinlock, but I don't know this code and ocfs2_mark_recovering_orphan_dir() looks tricky to me. (so some other eyes are much welcome)
>>   osb_lock is to protect osb filed such as 'osb_orphan_wipes', while
>> node_map_lock is to protect the node map 'osb_recovering_orphan_dirs'
>> specifically.
> 
> Thanks for this explanation.
> 
> But does "node_map_lock" really protects anything?
> It is just around some atomic function calls which shouldn't need any, right?
> 
> test_bit() is not documented as atomic, but {clear|set}_bit() could be executed just before or just after it with the current locking mechanism, so I don't really see how it would make a difference.
> 
> I don't understand the logic of this lock here.
> 
> Can you elaborate?

These code are introduced long time ago...
Refer to commit b4df6ed8db0c "[PATCH] ocfs2: fix orphan recovery
deadlock", I guess it plays a role 'barrier' and make sure test node map
is executed prior than signal orphan recovery thread. In other words, to
serialize evict inode and orphan recovery.

Thanks,
Joseph