group_cpus_evenly() could be part of storage driver's error handler, such
as nvme driver, when may happen during CPU hotplug, in which storage queue
has to drain its pending IOs because all CPUs associated with the queue
are offline and the queue is becoming inactive.  And handling IO needs
error handler to provide forward progress.

Then deadlock is caused:

1) inside CPU hotplug handler, CPU hotplug lock is held, and blk-mq's
   handler is waiting for inflight IO

2) error handler is waiting for CPU hotplug lock

3) inflight IO can't be completed in blk-mq's CPU hotplug handler
   because error handling can't provide forward progress.

Solve the deadlock by not holding CPU hotplug lock in group_cpus_evenly(),
in which two stage spreads are taken: 1) the 1st stage is over all present
CPUs; 2) the end stage is over all other CPUs.

Turns out the two stage spread just needs consistent 'cpu_present_mask',
and remove the CPU hotplug lock by storing it into one local cache.  This
way doesn't change correctness, because all CPUs are still covered.

Link: https://lkml.kernel.org/r/20231120083559.285174-1-ming.lei@redhat.com
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reported-by: Yi Zhang <yi.zhang@redhat.com>
Reported-by: Guangwu Zhang <guazhang@redhat.com>
Tested-by: Guangwu Zhang <guazhang@redhat.com>
Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Cc: Keith Busch <kbusch@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Export group_cpus_evenly() so that some modules
can make use of it to group CPUs evenly according
to NUMA and CPU locality.

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20230323053043.35-2-xieyongji@bytedance.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

allnoconfig grew these new build warnings in lib/group_cpus.c:

  lib/group_cpus.c:247:12: warning: ‘__group_cpus_evenly’ defined but not used [-Wunused-function]
  lib/group_cpus.c:75:13: warning: ‘build_node_to_cpumask’ defined but not used [-Wunused-function]
  lib/group_cpus.c:66:13: warning: ‘free_node_to_cpumask’ defined but not used [-Wunused-function]
  lib/group_cpus.c:43:23: warning: ‘alloc_node_to_cpumask’ defined but not used [-Wunused-function]

Widen the #ifdef CONFIG_SMP block to not expose unused helpers on
non-SMP builds.

Also annotate the preprocessor branches for better readability.

Fixes: f7b3ea8cf72f ("genirq/affinity: Move group_cpus_evenly() into lib/")
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20221227022905.352674-6-ming.lei@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

group_cpus_evenly() has become a generic function which can be used for
other subsystems than the interrupt subsystem, so move it into lib/.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jens Axboe <axboe@kernel.dk>                                                                                                                                                                                                    
Link: https://lore.kernel.org/r/20221227022905.352674-6-ming.lei@redhat.com