On Fri, Mar 31, 2023 at 10:50:45AM -0400, Waiman Long wrote:
> The current cpuset code uses the global cpuset_attach_old_cs variable
> to store the old cpuset value between consecutive cpuset_can_attach()
> and cpuset_attach() calls. Since a caller of cpuset_can_attach() may
> not need to hold the global cgroup_threadgroup_rwsem, parallel cpuset
> attach operations are possible.
>
> When there are concurrent cpuset attach operations in progress,
> cpuset_attach() may fetch the wrong value from cpuset_attach_old_cs
> causing incorrect result. To avoid this problem while still allowing
> certain level of parallelism, drop cpuset_attach_old_cs and use a
> per-cpuset attach_old_cs value. Also restrict to at most one active
> attach operation per cpuset to avoid corrupting the value of the
> per-cpuset attach_old_cs value.
>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
> kernel/cgroup/cpuset.c | 19 ++++++++++++++-----
> 1 file changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 2367de611c42..3f925c261513 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -198,6 +198,8 @@ struct cpuset {
>
> /* Handle for cpuset.cpus.partition */
> struct cgroup_file partition_file;
> +
> + struct cpuset *attach_old_cs;
> };
>
> /*
> @@ -2456,22 +2458,27 @@ static int fmeter_getrate(struct fmeter *fmp)
> return val;
> }
>
> -static struct cpuset *cpuset_attach_old_cs;
> -
> /* Called by cgroups to determine if a cpuset is usable; cpuset_rwsem held */
> static int cpuset_can_attach(struct cgroup_taskset *tset)
> {
> struct cgroup_subsys_state *css;
> - struct cpuset *cs;
> + struct cpuset *cs, *oldcs;
> struct task_struct *task;
> int ret;
>
> /* used later by cpuset_attach() */
> - cpuset_attach_old_cs = task_cs(cgroup_taskset_first(tset, &css));
> + oldcs = task_cs(cgroup_taskset_first(tset, &css));
> cs = css_cs(css);
>
> percpu_down_write(&cpuset_rwsem);
>
> + /*
> + * Only one cpuset attach operation is allowed for each cpuset.
> + */
> + ret = -EBUSY;
> + if (cs->attach_in_progress)
> + goto out_unlock;
That'll mean CLONE_INTO_CGROUP becomes even more interestig because it
isn't subject to this restriction in contrast to fork()+migrate, right?
