On 3/26/07, Srivatsa Vaddagiri <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]>
---
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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