[PATCH] Fix race between attach_task and cpuset_exit

Srivatsa Vaddagiri
2007-03-25T16:40:31Z

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@in.ibm.com>

---

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 2007-03-25 21:25:05.000000000 +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_tobe_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_tobe_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_tobe_released)
+ check_for_release(oldcs_tobe_released, ppathbuf);
return 0;
}

@@ -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).
@@ -2242,19 +2241,20 @@ void cpuset_exit(struct task_struct *tsk
{
struct cpuset *cs;

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

if (notify_on_release(cs)) {
char *pathbuf = NULL;

mutex_lock(&manage_mutex);
- if (atomic_dec_and_test(&cs->count))
+ if (!atomic_read(&cs->count))
check_for_release(cs, &pathbuf);
mutex_unlock(&manage_mutex);
cpuset_release_agent(pathbuf);
- } else {
- atomic_dec(&cs->count);
}
}

_

--
Regards,
vatsa