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

[Ming Lei <ming.lei@redhat.com>]

Hello Thomas,

On Wed, Aug 30, 2023 at 07:44:03PM +0200, Thomas Gleixner wrote:
> 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.

Good catch, we can annotate it with data_race(), and I think here the
readonly access to cpu_present_mask is fine, and we have many such usage,
such as for_each_present_cpu().

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

OK, I will document the change, such as:

There isn't difference compared with holding cpu hotplug lock, because the cpu
hp state is always changed even though cpu_present_mask isn't updated
with the lock. And all CPUs are always covered during the spread, the difference
is just that it is done in 1st stage or 2nd stage.

If you don't object, I will post next version with above change.


Thanks,
Ming