The following commit has been merged into the smp/urgent branch of tip:

Commit-ID: 64c71be97c02c3d3f24dea7c290912ad300538b9 Gitweb: https://git.kernel.org/tip/64c71be97c02c3d3f24dea7c290912ad300538b9 Author: Thomas Gleixner tglx@linutronix.de AuthorDate: Sat, 27 Mar 2021 22:01:36 +01:00 Committer: Thomas Gleixner tglx@linutronix.de CommitterDate: Sat, 19 Jun 2021 22:26:07 +02:00

cpu/hotplug: Cure the cpusets trainwreck

Alexey and Joshua tried to solve a cpusets related hotplug problem which is user space visible and results in unexpected behaviour for some time after a CPU has been plugged in and the corresponding uevent was delivered.

cpusets delegate the hotplug work (rebuilding cpumasks etc.) to a workqueue. This is done because the cpusets code has already a lock nesting of cgroups_mutex -> cpu_hotplug_lock. A synchronous callback or waiting for the work to finish with cpu_hotplug_lock held can and will deadlock because that results in the reverse lock order.

As a consequence the uevent can be delivered before cpusets have consistent state which means that a user space invocation of sched_setaffinity() to move a task to the plugged CPU fails up to the point where the scheduled work has been processed.

The same is true for CPU unplug, but that does not create user observable failure (yet).

It's still inconsistent to claim that an operation is finished before it actually is and that's the real issue at hand. uevents just make it reliably observable.

Obviously the problem should be fixed in cpusets/cgroups, but untangling that is pretty much impossible because according to the changelog of the commit which introduced this 8 years ago:

3a5a6d0c2b03("cpuset: don't nest cgroup_mutex inside get_online_cpus()")

the lock order cgroups_mutex -> cpu_hotplug_lock is a design decision and the whole code is built around that.

So bite the bullet and invoke the relevant cpuset function, which waits for the work to finish, in _cpu_up/down() after dropping cpu_hotplug_lock and only when tasks are not frozen by suspend/hibernate because that would obviously wait forever.

Waiting there with cpu_add_remove_lock, which is protecting the present and possible CPU maps, held is not a problem at all because neither work queues nor cpusets/cgroups have any lockchains related to that lock.

Waiting in the hotplug machinery is not problematic either because there are already state callbacks which wait for hardware queues to drain. It makes the operations slightly slower, but hotplug is slow anyway.

This ensures that state is consistent before returning from a hotplug up/down operation. It's still inconsistent during the operation, but that's a different story.

Add a large comment which explains why this is done and why this is not a dump ground for the hack of the day to work around half thought out locking schemes. Document also the implications vs. hotplug operations and serialization or the lack of it.

Thanks to Alexy and Joshua for analyzing why this temporary sched_setaffinity() failure happened.

Fixes: 3a5a6d0c2b03("cpuset: don't nest cgroup_mutex inside get_online_cpus()") Reported-by: Alexey Klimov aklimov@redhat.com Reported-by: Joshua Baker jobaker@redhat.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Tested-by: Alexey Klimov aklimov@redhat.com Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/87tuowcnv3.ffs@nanos.tec.linutronix.de --- kernel/cpu.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 49 insertions(+)

diff --git a/kernel/cpu.c b/kernel/cpu.c index e538518..eccc8cf 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -32,6 +32,7 @@ #include <linux/relay.h> #include <linux/slab.h> #include <linux/percpu-rwsem.h> +#include <linux/cpuset.h>

#include <trace/events/power.h> #define CREATE_TRACE_POINTS @@ -919,6 +920,52 @@ void clear_tasks_mm_cpumask(int cpu) rcu_read_unlock(); }

+/* + * + * Serialize hotplug trainwrecks outside of the cpu_hotplug_lock + * protected region. + * + * The operation is still serialized against concurrent CPU hotplug via + * cpu_add_remove_lock, i.e. CPU map protection. But it is _not_ + * serialized against other hotplug related activity like adding or + * removing of state callbacks and state instances, which invoke either the + * startup or the teardown callback of the affected state. + * + * This is required for subsystems which are unfixable vs. CPU hotplug and + * evade lock inversion problems by scheduling work which has to be + * completed _before_ cpu_up()/_cpu_down() returns. + * + * Don't even think about adding anything to this for any new code or even + * drivers. It's only purpose is to keep existing lock order trainwrecks + * working. + * + * For cpu_down() there might be valid reasons to finish cleanups which are + * not required to be done under cpu_hotplug_lock, but that's a different + * story and would be not invoked via this. + */ +static void cpu_up_down_serialize_trainwrecks(bool tasks_frozen) +{ + /* + * cpusets delegate hotplug operations to a worker to "solve" the + * lock order problems. Wait for the worker, but only if tasks are + * _not_ frozen (suspend, hibernate) as that would wait forever. + * + * The wait is required because otherwise the hotplug operation + * returns with inconsistent state, which could even be observed in + * user space when a new CPU is brought up. The CPU plug uevent + * would be delivered and user space reacting on it would fail to + * move tasks to the newly plugged CPU up to the point where the + * work has finished because up to that point the newly plugged CPU + * is not assignable in cpusets/cgroups. On unplug that's not + * necessarily a visible issue, but it is still inconsistent state, + * which is the real problem which needs to be "fixed". This can't + * prevent the transient state between scheduling the work and + * returning from waiting for it. + */ + if (!tasks_frozen) + cpuset_wait_for_hotplug(); +} + /* Take this CPU down. */ static int take_cpu_down(void *_param) { @@ -1108,6 +1155,7 @@ out: */ lockup_detector_cleanup(); arch_smt_update(); + cpu_up_down_serialize_trainwrecks(tasks_frozen); return ret; }

@@ -1302,6 +1350,7 @@ static int _cpu_up(unsigned int cpu, int tasks_frozen, enum cpuhp_state target) out: cpus_write_unlock(); arch_smt_update(); + cpu_up_down_serialize_trainwrecks(tasks_frozen); return ret; }