Re: [RFC PATCH 03/30] Lazy percpu counters

Kent Overstreet
2022-09-01T15:43:47Z

On Thu, Sep 01, 2022 at 10:48:39AM -0400, Steven Rostedt wrote:
> On Thu, 1 Sep 2022 10:32:19 -0400
> Kent Overstreet <kent.overstreet@linux.dev> wrote:
>
> > On Thu, Sep 01, 2022 at 08:51:31AM +0200, Peter Zijlstra wrote:
> > > On Tue, Aug 30, 2022 at 02:48:52PM -0700, Suren Baghdasaryan wrote:
> > > > +static void lazy_percpu_counter_switch_to_pcpu(struct raw_lazy_percpu_counter *c)
> > > > +{
> > > > + u64 __percpu *pcpu_v = alloc_percpu_gfp(u64, GFP_ATOMIC|__GFP_NOWARN);
> > >
> > > Realize that this is incorrect when used under a raw_spinlock_t.
> >
> > Can you elaborate?
>
> All allocations (including GFP_ATOMIC) grab normal spin_locks. When
> PREEMPT_RT is configured, normal spin_locks turn into a mutex, where as
> raw_spinlock's do not.
>
> Thus, if this is done within a raw_spinlock with PREEMPT_RT configured, it
> can cause a schedule while holding a spinlock.

Thanks, I think we should be good here but I'll document it anyways.