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

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Date:   Thu, 10 May 2018 14:49:44 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Larry Chen <lchen@...e.com>
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

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.