Re: [PATCH V4 resend] lib/group_cpus.c: avoid to acquire cpu hotplug lock in group_cpus_evenly

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 20 Nov 2023 12:00:59 -0800 Andrew Morton <akpm@linux-foundation.org> wrote:

> On Mon, 20 Nov 2023 16:35:59 +0800 Ming Lei <ming.lei@redhat.com> 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.
> 
> I'm not sure what is the intended merge path for this, but I can do lib/.
> 
> Do you think that a -stable backport is needed?  It sounds that way.
> 
> If so, are we able to identify a suitable Fixes: target?  That would
> predate f7b3ea8cf72f3 ("genirq/affinity: Move group_cpus_evenly() into
> lib/").  

No?  I think this predates 428e211641ed8 ("genirq/affinity: Replace
deprecated CPU-hotplug functions." also.

I'll slap a cc:stable on it and I'll let you and the -stable
maintainers figure it out.