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

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Message-ID: <f313cb6f-de75-2447-eebc-5c240bc243a2@wanadoo.fr>
Date:   Wed, 20 Jul 2022 10:26:35 +0200
From:   Marion & Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Joseph Qi <joseph.qi@...ux.alibaba.com>
Cc:     David.Laight@...LAB.COM, christophe.jaillet@...adoo.fr,
        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


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?

CJ


> Thanks,
> Joseph
>