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

[Ming Lei]

On Fri, Aug 18, 2023 at 10:01 PM Ming Lei <ming.lei@xxxxxxxxxx> 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.
>
> 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.
>
> Cc: Keith Busch <kbusch@xxxxxxxxxx>
> Cc: linux-nvme@xxxxxxxxxxxxxxxxxxx
> Cc: linux-block@xxxxxxxxxxxxxxx
> Reported-by: Yi Zhang <yi.zhang@xxxxxxxxxx>
> Reported-by: Guangwu Zhang <guazhang@xxxxxxxxxx>
> Tested-by: Guangwu Zhang <guazhang@xxxxxxxxxx>
> Reviewed-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> ---
> V3:
>         - reuse `npresmsk`, and avoid to allocate new variable, suggested by
>         Chengming Zhou

Hello Thomas and Jens,

Ping...

Thanks,
Ming