linux-kernel - Re: [PATCH] ocfs2: ocfs2_inode_lock_tracker does not distinguish lock level

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Date:   Fri, 11 May 2018 12:16:51 +0800
From:   Larry Chen <lchen@...e.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     mfasheh@...sity.com, jlbec@...lplan.org,
        linux-kernel@...r.kernel.org, ocfs2-devel@....oracle.com
Subject: Re: [PATCH] ocfs2: ocfs2_inode_lock_tracker does not distinguish lock
 level

Hello Andrew,


On 05/11/2018 05:49 AM, Andrew Morton wrote:
> On Thu, 10 May 2018 13:32:30 +0800 Larry Chen <lchen@...e.com> wrote:
>
>> ocfs2_inode_lock_tracker as a variant of ocfs2_inode_lock,
>> is used to prevent deadlock due to recursive lock acquisition.
>>
>> But this function does not distinguish
>> whether the requested level is EX or PR.
>>
>> If a RP lock has been attained, this function
>> will immediately return success afterwards even
>> an EX lock is requested.
>>
>> But actually the return value does not mean that
>> the process got a EX lock, because ocfs2_inode_lock
>> has not been called.
>>
>> When taking lock levels into account, we face some different situations.
>> 1. no lock is held
>>     In this case, just lock the inode and return 0
>>
>> 2. We are holding a lock
>>     For this situation, things diverges into several cases
>>
>>     wanted     holding	     what to do
>>     ex		ex	    see 2.1 below
>>     ex		pr	    see 2.2 below
>>     pr		ex	    see 2.1 below
>>     pr		pr	    see 2.1 below
>>
>>     2.1 lock level that is been held is compatible
>>     with the wanted level, so no lock action will be tacken.
>>
>>     2.2 Otherwise, an upgrade is needed, but it is forbidden.
>>
>> Reason why upgrade within a process is forbidden is that
>> lock upgrade may cause dead lock. The following illustrate
>> how it happens.
>>
>>          process 1                             process 2
>> ocfs2_inode_lock_tracker(ex=0)
>>                                 <======   ocfs2_inode_lock_tracker(ex=1)
>>
>> ocfs2_inode_lock_tracker(ex=1)
>>
> Nice changelog, but it gives no information about the severity of the
> bug: how often does it hit and what is the end-user impact.
>
> This info is needed so that I and others can decide which kernel
> version(s) need the patch, so please always include it when fixing a
> bug, thanks.

Thanks for your review and feel sorry for not providing enough information.

For the status quo of ocfs2, without this patch, neither a bug nor end-user
impact will be caused because the wrong logic is avoided.

But I'm afraid this generic interface, may be called by other
developers in future and used in this situation.

     a process
ocfs2_inode_lock_tracker(ex=0)
ocfs2_inode_lock_tracker(ex=1)

By the way, should I resend this patch with this info included?

Thanks
Larry

>