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Re: [libvirt] [PATCH] cgroup: be robust against cgroup movement races



On Mon, May 20, 2013 at 09:07:17PM -0600, Eric Blake wrote:
> https://bugzilla.redhat.com/show_bug.cgi?id=965169 documents a
> problem starting domains when cgroups are enabled; I was able
> to reliably reproduce the race about 5% of the time when I added
> hooks to domain startup by 3 seconds (as that seemed to be about
> the length of time that qemu created and then closed a temporary
> thread, probably related to aio handling of initially opening
> a disk image).
> 
> There are some inherent TOCTTOU races when moving tasks between
> kernel cgroups, precisely because threads can be created or
> completed in the window between when we read a thread id from the
> source and when we write to the destination.  As the goal of
> virCgroupMoveTask is merely to move ALL tasks into the new
> cgroup, it is sufficient to iterate until no more threads are
> being created in the old group, and ignoring any threads that
> die before we can move them.
> 
> It would be nicer to start the threads in the right cgroup to
> begin with, but by default, all child threads are created in
> the same cgroup as their parent, and we don't want vcpu child
> threads in the emulator cgroup, so I don't see any good way
> of avoiding the move.  It would also be nice if the kernel were
> to implement something like rename() as a way to atomically move
> a group of threads from one cgroup to another, instead of forcing
> a window where we have to read and parse the source, then format
> and write back into the destination.
> 
> * src/util/vircgroup.c (virCgroupAddTaskStrController): Ignore
> ESRCH, because a thread ended between read and write attempts.
> (virCgroupMoveTask): Loop until all threads have moved.
> 
> Signed-off-by: Eric Blake <eblake redhat com>
> ---
>  src/util/vircgroup.c | 28 ++++++++++++++++++++--------
>  1 file changed, 20 insertions(+), 8 deletions(-)
> 
> diff --git a/src/util/vircgroup.c b/src/util/vircgroup.c
> index 07ea2c3..6d44694 100644
> --- a/src/util/vircgroup.c
> +++ b/src/util/vircgroup.c
> @@ -1037,7 +1037,11 @@ static int virCgroupAddTaskStrController(virCgroupPtr group,
>              goto cleanup;
> 
>          rc = virCgroupAddTaskController(group, p, controller);
> -        if (rc != 0)
> +        /* A thread that exits between when we first read the source
> +         * tasks and now is not fatal.  */
> +        if (rc == -ESRCH)
> +            rc = 0;
> +        else if (rc != 0)
>              goto cleanup;
> 
>          next = strchr(cur, '\n');
> @@ -1074,15 +1078,23 @@ int virCgroupMoveTask(virCgroupPtr src_group, virCgroupPtr dest_group)
>              !dest_group->controllers[i].mountPoint)
>              continue;
> 
> -        rc = virCgroupGetValueStr(src_group, i, "tasks", &content);
> -        if (rc != 0)
> -            return rc;
> +        /* New threads are created in the same group as their parent;
> +         * but if a thread is created after we first read we aren't
> +         * aware that it needs to move.  Therefore, we must iterate
> +         * until content is empty.  */
> +        while (1) {
> +            rc = virCgroupGetValueStr(src_group, i, "tasks", &content);
> +            if (rc != 0)
> +                return rc;
> +            if (!*content)
> +                break;
> 
> -        rc = virCgroupAddTaskStrController(dest_group, content, i);
> -        if (rc != 0)
> -            goto cleanup;
> +            rc = virCgroupAddTaskStrController(dest_group, content, i);
> +            if (rc != 0)
> +                goto cleanup;
> 
> -        VIR_FREE(content);
> +            VIR_FREE(content);
> +        }
>      }
> 
>  cleanup:

ACK, this looks like the best we can do here.


Daniel
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