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SubjectRe: [PATCH 2/3] ocfs2: Remove a useless spinlock
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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
>

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