The following commit has been merged into the sched/core branch of tip:

Commit-ID: 406100f3da08066c00105165db8520bbc7694a36 Gitweb: https://git.kernel.org/tip/406100f3da08066c00105165db8520bbc7694a36 Author: Daniel Jordan daniel.m.jordan@oracle.com AuthorDate: Thu, 12 Nov 2020 12:17:11 -05:00 Committer: Peter Zijlstra peterz@infradead.org CommitterDate: Thu, 19 Nov 2020 11:25:45 +01:00

cpuset: fix race between hotplug work and later CPU offline

One of our machines keeled over trying to rebuild the scheduler domains. Mainline produces the same splat:

BUG: unable to handle page fault for address: 0000607f820054db CPU: 2 PID: 149 Comm: kworker/1:1 Not tainted 5.10.0-rc1-master+ #6 Workqueue: events cpuset_hotplug_workfn RIP: build_sched_domains Call Trace: partition_sched_domains_locked rebuild_sched_domains_locked cpuset_hotplug_workfn

It happens with cgroup2 and exclusive cpusets only. This reproducer triggers it on an 8-cpu vm and works most effectively with no preexisting child cgroups:

cd $UNIFIED_ROOT mkdir cg1 echo 4-7 > cg1/cpuset.cpus echo root > cg1/cpuset.cpus.partition

# with smt/control reading 'on', echo off > /sys/devices/system/cpu/smt/control

RIP maps to

sd->shared = *per_cpu_ptr(sdd->sds, sd_id);

from sd_init(). sd_id is calculated earlier in the same function:

cpumask_and(sched_domain_span(sd), cpu_map, tl->mask(cpu)); sd_id = cpumask_first(sched_domain_span(sd));

tl->mask(cpu), which reads cpu_sibling_map on x86, returns an empty mask and so cpumask_first() returns >= nr_cpu_ids, which leads to the bogus value from per_cpu_ptr() above.

The problem is a race between cpuset_hotplug_workfn() and a later offline of CPU N. cpuset_hotplug_workfn() updates the effective masks when N is still online, the offline clears N from cpu_sibling_map, and then the worker uses the stale effective masks that still have N to generate the scheduling domains, leading the worker to read N's empty cpu_sibling_map in sd_init().

rebuild_sched_domains_locked() prevented the race during the cgroup2 cpuset series up until the Fixes commit changed its check. Make the check more robust so that it can detect an offline CPU in any exclusive cpuset's effective mask, not just the top one.

Fixes: 0ccea8feb980 ("cpuset: Make generate_sched_domains() work with partition") Signed-off-by: Daniel Jordan daniel.m.jordan@oracle.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Acked-by: Tejun Heo tj@kernel.org Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/20201112171711.639541-1-daniel.m.jordan@oracle.com --- kernel/cgroup/cpuset.c | 33 ++++++++++++++++++++++++++++----- 1 file changed, 28 insertions(+), 5 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c index 57b5b5d..53c70c4 100644 --- a/kernel/cgroup/cpuset.c +++ b/kernel/cgroup/cpuset.c @@ -983,25 +983,48 @@ partition_and_rebuild_sched_domains(int ndoms_new, cpumask_var_t doms_new[], */ static void rebuild_sched_domains_locked(void) { + struct cgroup_subsys_state *pos_css; struct sched_domain_attr *attr; cpumask_var_t *doms; + struct cpuset *cs; int ndoms;

lockdep_assert_cpus_held(); percpu_rwsem_assert_held(&cpuset_rwsem);

/* - * We have raced with CPU hotplug. Don't do anything to avoid + * If we have raced with CPU hotplug, return early to avoid * passing doms with offlined cpu to partition_sched_domains(). - * Anyways, hotplug work item will rebuild sched domains. + * Anyways, cpuset_hotplug_workfn() will rebuild sched domains. + * + * With no CPUs in any subpartitions, top_cpuset's effective CPUs + * should be the same as the active CPUs, so checking only top_cpuset + * is enough to detect racing CPU offlines. */ if (!top_cpuset.nr_subparts_cpus && !cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask)) return;

- if (top_cpuset.nr_subparts_cpus && - !cpumask_subset(top_cpuset.effective_cpus, cpu_active_mask)) - return; + /* + * With subpartition CPUs, however, the effective CPUs of a partition + * root should be only a subset of the active CPUs. Since a CPU in any + * partition root could be offlined, all must be checked. + */ + if (top_cpuset.nr_subparts_cpus) { + rcu_read_lock(); + cpuset_for_each_descendant_pre(cs, pos_css, &top_cpuset) { + if (!is_partition_root(cs)) { + pos_css = css_rightmost_descendant(pos_css); + continue; + } + if (!cpumask_subset(cs->effective_cpus, + cpu_active_mask)) { + rcu_read_unlock(); + return; + } + } + rcu_read_unlock(); + }

/* Generate domain masks and attrs */ ndoms = generate_sched_domains(&doms, &attr);