Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <07c924de-78bf-c993-ce73-635af71f4edd@linux.alibaba.com>
Date:   Wed, 20 Jul 2022 09:59:47 +0800
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:91.0)
 Gecko/20100101 Thunderbird/91.11.0
Subject: Re: [PATCH 2/3] ocfs2: Remove a useless spinlock
Content-Language: en-US
To:     Christophe JAILLET <christophe.jaillet@wanadoo.fr>,
        David Laight <David.Laight@ACULAB.COM>,
        Mark Fasheh <mark@fasheh.com>, Joel Becker <jlbec@evilplan.org>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kernel-janitors@vger.kernel.org" <kernel-janitors@vger.kernel.org>,
        "ocfs2-devel@oss.oracle.com" <ocfs2-devel@oss.oracle.com>
References: <bd6796635e58f9c47cf857573c3b9474a00ce26a.1658224839.git.christophe.jaillet@wanadoo.fr>
 <8ba7004d330cbe5f626539a8a3bff696d0c4285e.1658224839.git.christophe.jaillet@wanadoo.fr>
 <7b644e5d32d74d3d90dfc5b1786ae5b9@AcuMS.aculab.com>
 <29c3fbdd-7695-46c5-bb75-fe358c574ab3@wanadoo.fr>
From:   Joseph Qi <joseph.qi@linux.alibaba.com>
In-Reply-To: <29c3fbdd-7695-46c5-bb75-fe358c574ab3@wanadoo.fr>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Precedence: bulk


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