Re: [PATCH] Fix race between attach_task and cpuset_exit

[Paul Menage]

On 3/26/07, Srivatsa Vaddagiri <vatsa@xxxxxxxxxx> wrote:
> On Sun, Mar 25, 2007 at 12:50:25PM -0700, Paul Jackson wrote:
> > Is there perhaps another race here?
>
> Yes, we have!
>
> Modified patch below. Compile/boot tested on a x86_64 box.
>
>
> Currently cpuset_exit() changes the exiting task's ->cpuset pointer w/o
> taking task_lock(). This can lead to ugly races between attach_task and
> cpuset_exit. Details of the races are described at
> http://lkml.org/lkml/2007/3/24/132.
>
> Patch below closes those races. It is against 2.6.21-rc4 and has
> undergone a simple compile/boot test on a x86_64 box.
>
> Signed-off-by : Srivatsa Vaddagiri <vatsa@xxxxxxxxxx>
>
>
> ---
>
>
> diff -puN kernel/cpuset.c~cpuset_race_fix kernel/cpuset.c
> --- linux-2.6.21-rc4/kernel/cpuset.c~cpuset_race_fix    2007-03-25 21:08:27.000000000 +0530
> +++ linux-2.6.21-rc4-vatsa/kernel/cpuset.c      2007-03-26 16:48:24.000000000 +0530
> @@ -1182,6 +1182,7 @@ static int attach_task(struct cpuset *cs
>         pid_t pid;
>         struct task_struct *tsk;
>         struct cpuset *oldcs;
> +       struct cpuset *oldcs_to_be_released = NULL;
>         cpumask_t cpus;
>         nodemask_t from, to;
>         struct mm_struct *mm;
> @@ -1237,6 +1238,8 @@ static int attach_task(struct cpuset *cs
>         }
>         atomic_inc(&cs->count);
>         rcu_assign_pointer(tsk->cpuset, cs);
> +       if (atomic_dec_and_test(&oldcs->count))
> +               oldcs_to_be_released = oldcs;
>         task_unlock(tsk);
>
>         guarantee_online_cpus(cs, &cpus);
> @@ -1257,8 +1260,8 @@ static int attach_task(struct cpuset *cs
>
>         put_task_struct(tsk);
>         synchronize_rcu();
> -       if (atomic_dec_and_test(&oldcs->count))
> -               check_for_release(oldcs, ppathbuf);
> +       if (oldcs_to_be_released)
> +               check_for_release(oldcs_to_be_released, ppathbuf);
>         return 0;
>  }

Is this part of the patch necessary? If we're adding a task_lock() in
cpuset_exit(), then the problem that Vatsa described (both
cpuset_attach_task() and cpuset_exit() decrementing the same cpuset
count, and cpuset_attach_task() incrementing the count on a cpuset
that the task doesn't eventually end up in) go away, since only one
thread will retrieve the old value of the task's cpuset in order to
decrement its count.


> @@ -2200,10 +2203,6 @@ void cpuset_fork(struct task_struct *chi
>   * it is holding that mutex while calling check_for_release(),
>   * which calls kmalloc(), so can't be called holding callback_mutex().
>   *
> - * We don't need to task_lock() this reference to tsk->cpuset,
> - * because tsk is already marked PF_EXITING, so attach_task() won't
> - * mess with it, or task is a failed fork, never visible to attach_task.
> - *
>   * the_top_cpuset_hack:
>   *
>   *    Set the exiting tasks cpuset to the root cpuset (top_cpuset).
> @@ -2241,20 +2240,23 @@ void cpuset_fork(struct task_struct *chi
>  void cpuset_exit(struct task_struct *tsk)
>  {
>         struct cpuset *cs;
> +       struct cpuset *oldcs_to_be_released = NULL;
>
> +       task_lock(tsk);
>         cs = tsk->cpuset;
>         tsk->cpuset = &top_cpuset;      /* the_top_cpuset_hack - see above */
> +       if (atomic_dec_and_test(&cs->count))
> +               oldcs_to_be_released = cs;
> +       task_unlock(tsk);

I think this is still racy - at this point we're holding a reference
on a cpuset that could have a zero count, and we don't hold
manage_mutex or callback_mutex. So a concurrent rmdir could zap the
directory and free the cpuset.

Shouldn't we just put a task_lock()/task_unlock() around these lines
and leave everything else as-is?

	task_lock(tsk);
	cs = tsk->cpuset;
	tsk->cpuset = &top_cpuset;	/* the_top_cpuset_hack - see above */
	task_unlock(tsk)

Paul
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