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

[sukadev at us.ibm.com]

Serge E. Hallyn [serue at us.ibm.com] wrote:
| 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!

And mine too.  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>
Tested-by: Sukadev Bhattiprolu <sukadev at us.ibm.com>
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