Re: [PATCH v2] cpuset: fix race between hotplug work and later CPU offline

[Peter Zijlstra]

On Thu, Nov 12, 2020 at 12:17:11PM -0500, Daniel Jordan wrote:
> 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@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Li Zefan <lizefan@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Prateek Sood <prsood@xxxxxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Waiman Long <longman@xxxxxxxxxx>
> Cc: cgroups@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx

Works for me. TJ, do I take this or do you want it in the cgroup tree?

In that case:

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>