Re: [PATCH 3/3] cgroup/cpuset: Allow only one active attach operation per cpuset

[Waiman Long]

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 that in latter version.

Cheers,
Longman