[PATCH V3] lib/group_cpus.c: avoid to acquire cpu hotplug lock in group_cpus_evenly

[Thomas Gleixner]

Ming!

On Fri, Aug 18 2023 at 22:01, Ming Lei wrote:

> 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 dead lock 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.

That solves the deadlock, but makes the code racy against a concurrent
hot-add operation which modifies cpu_present_mask. IOW, it introduces a
data race.

The changelog does not explain why this does not matter nor does the
comment.

Thanks,

        tglx