Re: [PATCH v2 22/35] percpu-rwsem: enable percpu_sem destruction in atomic context

[Hillf Danton <hdanton@sina.com>]

On Mon, 31 Jan 2022 10:04:16 -0800 Suren Baghdasaryan wrote:
> On Sat, Jan 29, 2022 at 4:13 AM Hillf Danton wrote:
> >
> > On Fri, 28 Jan 2022 05:09:53 -0800 Michel Lespinasse wrote:
> > > +
> > > +static LIST_HEAD(destroy_list);
> > > +static DEFINE_SPINLOCK(destroy_list_lock);
> >
> >    static bool destroyer_running;
> >
> > > +
> > > +static void destroy_list_workfn(struct work_struct *work)
> > > +{
> > > +     struct percpu_rw_semaphore *sem, *sem2;
> > > +     LIST_HEAD(to_destroy);
> > > +
> >
> > again:
> >
> > > +     spin_lock(&destroy_list_lock);
> >
> >         if (list_empty(&destroy_list)) {
> >                 destroyer_running = false;
> >                 spin_unlock(&destroy_list_lock);
> >                 return;
> >         }
> >         destroyer_running = true;
> >
> > > +     list_splice_init(&destroy_list, &to_destroy);
> > > +     spin_unlock(&destroy_list_lock);
> > > +
> > > +     if (list_empty(&to_destroy))
> > > +             return;
> > > +
> > > +     list_for_each_entry_safe(sem, sem2, &to_destroy, destroy_list_entry) {
> >
> >                 list_del(&sem->destroy_list_entry);
> >
> > > +             percpu_free_rwsem(sem);
> > > +             kfree(sem);
> > > +     }
> >
> >         goto again;
> > > +}
> > > +
> > > +static DECLARE_WORK(destroy_list_work, destroy_list_workfn);
> > > +
> > > +void percpu_rwsem_async_destroy(struct percpu_rw_semaphore *sem)
> > > +{
> > > +     spin_lock(&destroy_list_lock);
> > > +     list_add_tail(&sem->destroy_list_entry, &destroy_list);
> > > +     spin_unlock(&destroy_list_lock);
> > > +     schedule_work(&destroy_list_work);
> >
> > Nits
> >         spin_lock(&destroy_list_lock);
> > 1/      /* LIFO */
> >         list_add(&sem->destroy_list_entry, &destroy_list);
> > 2/      /* spawn worker if it is idle */
> >         if (!destroyer_running)
> > 3/              /* this is not critical work */
> >                 queue_work(system_unbound_wq, &destroy_list_work);
> >         spin_unlock(&destroy_list_lock);
> 
> Thanks for the review! Just to clarify, are you suggesting
> simplifications to the current patch or do you see a function issue?

Apart from the nits that can be safely ignored in usual spins, I wonder if
the async destroy can be used in the contexts wrt raw_spin_lock.

Hillf

	raw_spin_lock_irq(&foo->lock);
	...
	percpu_rwsem_async_destroy(*sem);
	...
	raw_spin_unlock_irq(&foo->lock);