linux-kernel - Re: [PATCH] cgroup: fix reverse unlock sequence in cgroup_get

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Date:	Tue, 21 Jul 2009 17:31:06 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Zefan Li <lizf.kernel@...il.com>
Cc:	Xiaotian Feng <dfeng@...hat.com>, menage@...gle.com,
	lizf@...fujitsu.com, containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cgroup: fix reverse unlock sequence in cgroup_get_sb

* Zefan Li <lizf.kernel@...il.com> [2009-07-21 19:38:03]:

> 2009/7/21, Balbir Singh <balbir@...ux.vnet.ibm.com>:
> >
> > * Xiaotian Feng <dfeng@...hat.com> [2009-07-21 18:25:26]:
> >
> > > In cgroup_get_sb, the lock sequence is:
> > >       mutex_lock(&inode->i_mutex);
> > >       mutex_lock(&cgroup->mutex);
> > > so the last unlock sequence should be:
> > >       mutex_unlock(&cgroup->mutex);
> > >       mutex_unlock(&inode->i_mutex);
> > >
> > > Signed-off-by: Xiaotian Feng <dfeng@...hat.com>
> > > ---
> > >  kernel/cgroup.c |    2 +-
> > >  1 files changed, 1 insertions(+), 1 deletions(-)
> > >
> > > diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> > > index 3737a68..11ef162 100644
> > > --- a/kernel/cgroup.c
> > > +++ b/kernel/cgroup.c
> > > @@ -1140,8 +1140,8 @@ static int cgroup_get_sb(struct file_system_type
> > *fs_type,
> > >               BUG_ON(root->number_of_cgroups != 1);
> > >
> > >               cgroup_populate_dir(root_cgrp);
> > > -             mutex_unlock(&inode->i_mutex);
> > >               mutex_unlock(&cgroup_mutex);
> > > +             mutex_unlock(&inode->i_mutex);
> > >       }
> > >
> >
> > Seems reasonable to me. You might also want to mention that elsewhere
> > the sequence is unlock cgroup_mutex followed by inode->i_mutex.
> >
> > Acked-by: Balbir Singh balbir@...ux.vnet.ibm.com
> 
> 
> No, the unlock order is irrelevant. It's the lock order that matters. So
> this patch
> fixes nothing.
> 
> Xiaotian, you didn't run into deadlock, did you?
>


Li, Consider the following


lock(A)
lock(B)
unlock(A) 
unlock(B)

Tomorrow if a unsuspecting programmer does this

lock(A)
lock(B)
unlock(A) 

code block 

unlock(B)


What protects code block? lock B? Is that the intention?

-- 
	Balbir
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