[Devel] Re: [BUG]: Crash with CONFIG_FAIR_CGROUP_SCHED=y

[Serge E. Hallyn]

Quoting Srivatsa Vaddagiri (vatsa at linux.vnet.ibm.com):
> 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 <vatsa at linux.vnet.ibm.com>

Tested-by: Serge Hallyn <serue at us.ibm.com>

> 
> ---
>  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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