On Fri, Apr 07, 2023 at 11:30:14AM -0400, Waiman Long wrote:
> On 4/6/23 05:44, Christian Brauner wrote:
> > 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?
>
> This patch is not related to the CLONE_INTO_CGROUP. In fact, I have dropped
I know that. My point had been that this patch introduced a new
restriction affecting parallelism in the migration path while the
CLONE_INTO_CGROUP path remained unaffected.
