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

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Message-ID: <07c924de-78bf-c993-ce73-635af71f4edd@linux.alibaba.com>
Date:   Wed, 20 Jul 2022 09:59:47 +0800
From:   Joseph Qi <joseph.qi@...ux.alibaba.com>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        David Laight <David.Laight@...LAB.COM>,
        Mark Fasheh <mark@...heh.com>, Joel Becker <jlbec@...lplan.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>,
        "ocfs2-devel@....oracle.com" <ocfs2-devel@....oracle.com>
Subject: Re: [PATCH 2/3] ocfs2: Remove a useless spinlock



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,
Joseph