linux-kernel - Re: [RFC][PATCH 1/3] CGroups: Add a per-subsystem hierarchy

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Message-ID: <6599ad830812101644l639daab5yab35e299d3badad9@mail.gmail.com>
Date:	Wed, 10 Dec 2008 16:44:51 -0800
From:	Paul Menage <menage@...gle.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	balbir@...ux.vnet.ibm.com, containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [RFC][PATCH 1/3] CGroups: Add a per-subsystem hierarchy_mutex

On Wed, Dec 10, 2008 at 4:37 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
>> +static void cgroup_unlock_hierarchy(struct cgroupfs_root *root)
>> +{
>> +     int i;
>> +
>> +     for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
>> +             struct cgroup_subsys *ss = subsys[i];
>> +             if (ss->root == root)
>> +                     mutex_unlock(&ss->hierarchy_mutex);
>> +     }
>> +}
>> +
> Maybe no problem..but I don't like releasing lock in the order of acquiring lock.
>
>        for (i = CGROUP_SUBSYS_COUNT - 1; i >=0; i--)  ?

The order that you release the locks is irrelevant for correctness. In
this case, since the only callers of cgroup_lock_hierarchy() also hold
cgroup_mutex and hence can't race with one another, the order of
locking is irrelevant for correctness too - right now the locking
order is just designed to keep lockdep happy.

I think that the reverse-ordered loop is less readable for no gain.

Paul
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