linux-kernel - Re: [PATCH 2/2] cgroup: if you list_empty() a head then don't list

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Message-ID: <AANLkTi=P0xPXs+_6KGa7SBfy_aNcq791tbbvOaKfen8J@mail.gmail.com>
Date:	Tue, 15 Mar 2011 09:20:00 -0700
From:	Paul Menage <menage@...gle.com>
To:	Phil Carmody <ext-phil.2.carmody@...ia.com>
Cc:	lizf@...fujitsu.com, containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH 2/2] cgroup: if you list_empty() a head then don't
 list_del() it

On Tue, Mar 15, 2011 at 6:08 AM, Phil Carmody
<ext-phil.2.carmody@...ia.com> wrote:
> list_del() leaves poison in the prev and next pointers. The next
> list_empty() will compare those poisons, and say the list isn't empty.
> Any list operations that assume the node is on a list because of such a
> check will be fooled into dereferencing poison. One needs to INIT the
> node after the del, and fortunately there's already a wrapper for that -
> list_del_init().
>
> Some of the dels are followed by deallocations, so can be ignored,
> and one can be merged with an add to make a move. Apart from that, I
> erred on the side of caution in making nodes list_empty()-queriable.
>
> Signed-off-by: Phil Carmody <ext-phil.2.carmody@...ia.com>

Reviewed-by: Paul Menage <menage@...gle.com>

Thanks,
Paul

> ---
>  kernel/cgroup.c |   14 ++++++--------
>  1 files changed, 6 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index b24d702..bcc7336 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1813,10 +1813,8 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
>
>        /* Update the css_set linked lists if we're using them */
>        write_lock(&css_set_lock);
> -       if (!list_empty(&tsk->cg_list)) {
> -               list_del(&tsk->cg_list);
> -               list_add(&tsk->cg_list, &newcg->tasks);
> -       }
> +       if (!list_empty(&tsk->cg_list))
> +               list_move(&tsk->cg_list, &newcg->tasks);
>        write_unlock(&css_set_lock);
>
>        for_each_subsys(root, ss) {
> @@ -3655,12 +3653,12 @@ again:
>        spin_lock(&release_list_lock);
>        set_bit(CGRP_REMOVED, &cgrp->flags);
>        if (!list_empty(&cgrp->release_list))
> -               list_del(&cgrp->release_list);
> +               list_del_init(&cgrp->release_list);
>        spin_unlock(&release_list_lock);
>
>        cgroup_lock_hierarchy(cgrp->root);
>        /* delete this cgroup from parent->children */
> -       list_del(&cgrp->sibling);
> +       list_del_init(&cgrp->sibling);
>        cgroup_unlock_hierarchy(cgrp->root);
>
>        d = dget(cgrp->dentry);
> @@ -3879,7 +3877,7 @@ void cgroup_unload_subsys(struct cgroup_subsys *ss)
>        subsys[ss->subsys_id] = NULL;
>
>        /* remove subsystem from rootnode's list of subsystems */
> -       list_del(&ss->sibling);
> +       list_del_init(&ss->sibling);
>
>        /*
>         * disentangle the css from all css_sets attached to the dummytop. as
> @@ -4253,7 +4251,7 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks)
>        if (!list_empty(&tsk->cg_list)) {
>                write_lock(&css_set_lock);
>                if (!list_empty(&tsk->cg_list))
> -                       list_del(&tsk->cg_list);
> +                       list_del_init(&tsk->cg_list);
>                write_unlock(&css_set_lock);
>        }
>
> --
> 1.7.2.rc1.37.gf8c40
>
>
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