Quoting Srivatsa Vaddagiri ([email protected]):
> On Fri, Nov 09, 2007 at 09:45:21AM +0100, Dmitry Adamushko wrote:
> > Humm... the 'current' is not kept within the tree but
> > current->se.on_rq is supposed to be '1' ,
> > so the old code looks ok to me (at least for the 'leaf' elements).
>
> You are damned right! Sorry my mistake with the previous analysis and
> (as I now find out) testing :(
>
> There are couple of problems discovered by Suka's test:
>
> - The test requires the cgroup filesystem to be mounted with
> atleast the cpu and ns options (i.e both namespace and cpu
> controllers are active in the same hierarchy).
>
> # mkdir /dev/cpuctl
> # mount -t cgroup -ocpu,ns none cpuctl
> (or simply)
> # mount -t cgroup none cpuctl -> Will activate all controllers
> in same hierarchy.
>
> - The test invokes clone() with CLONE_NEWNS set. This causes a a new child
> to be created, also a new group (do_fork->copy_namespaces->ns_cgroup_clone->
> cgroup_clone) and the child is attached to the new group (cgroup_clone->
> attach_task->sched_move_task). At this point in time, the child's scheduler
> related fields are uninitialized (including its on_rq field, which it has
> inherited from parent). As a result sched_move_task thinks its on
> runqueue, when it isn't.
>
> As a solution to this problem, I moved sched_fork() call, which
> initializes scheduler related fields on a new task, before
> copy_namespaces(). I am not sure though whether moving up will
> cause other side-effects. Do you see any issue?
>
> - The second problem exposed by this test is that task_new_fair()
> assumes that parent and child will be part of the same group (which
> needn't be as this test shows). As a result, cfs_rq->curr can be NULL
> for the child.
>
> The solution is to test for curr pointer being NULL in
> task_new_fair().
>
>
> With the patch below, I could run ns_exec() fine w/o a crash.
>
> Suka, can you verify whether this patch fixes your problem?
Works on my machine. Thanks!
> --
>
> Fix copy_namespace() <-> sched_fork() dependency in do_fork, by moving
> up sched_fork().
>
> Also introduce a NULL pointer check for 'curr' in task_new_fair().
>
> Signed-off-by : Srivatsa Vaddagiri <[email protected]>
Tested-by: Serge Hallyn <[email protected]>
>
> ---
> kernel/fork.c | 6 +++---
> kernel/sched_fair.c | 2 +-
> 2 files changed, 4 insertions(+), 4 deletions(-)
>
> Index: current/kernel/fork.c
> ===================================================================
> --- current.orig/kernel/fork.c
> +++ current/kernel/fork.c
> @@ -1121,6 +1121,9 @@ static struct task_struct *copy_process(
> p->blocked_on = NULL; /* not blocked yet */
> #endif
>
> + /* Perform scheduler related setup. Assign this task to a CPU. */
> + sched_fork(p, clone_flags);
> +
> if ((retval = security_task_alloc(p)))
> goto bad_fork_cleanup_policy;
> if ((retval = audit_alloc(p)))
> @@ -1210,9 +1213,6 @@ static struct task_struct *copy_process(
> INIT_LIST_HEAD(&p->ptrace_children);
> INIT_LIST_HEAD(&p->ptrace_list);
>
> - /* Perform scheduler related setup. Assign this task to a CPU. */
> - sched_fork(p, clone_flags);
> -
> /* Now that the task is set up, run cgroup callbacks if
> * necessary. We need to run them before the task is visible
> * on the tasklist. */
> Index: current/kernel/sched_fair.c
> ===================================================================
> --- current.orig/kernel/sched_fair.c
> +++ current/kernel/sched_fair.c
> @@ -1023,7 +1023,7 @@ static void task_new_fair(struct rq *rq,
> place_entity(cfs_rq, se, 1);
>
> if (sysctl_sched_child_runs_first && this_cpu == task_cpu(p) &&
> - curr->vruntime < se->vruntime) {
> + curr && curr->vruntime < se->vruntime) {
> /*
> * Upon rescheduling, sched_class::put_prev_task() will place
> * 'current' within the tree based on its new key value.
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