linux-kernel - Re: [RFT][PATCH v2 2/4] ACPI: OSL: Add support for deferred unmapping of ACPI memory

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Date:   Mon, 22 Jun 2020 17:27:53 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Dan Williams <dan.j.williams@...el.com>,
        Erik Kaneda <erik.kaneda@...el.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Len Brown <lenb@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Ira Weiny <ira.weiny@...el.com>,
        James Morse <james.morse@....com>,
        Myron Stowe <myron.stowe@...hat.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Bob Moore <robert.moore@...el.com>
Subject: Re: [RFT][PATCH v2 2/4] ACPI: OSL: Add support for deferred unmapping
 of ACPI memory

On Mon, Jun 22, 2020 at 4:56 PM Andy Shevchenko
<andy.shevchenko@...il.com> wrote:
>
> On Mon, Jun 22, 2020 at 5:06 PM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> >
> > From: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
> >
> > Implement acpi_os_unmap_deferred() and
> > acpi_os_release_unused_mappings() and set ACPI_USE_DEFERRED_UNMAPPING
> > to allow ACPICA to use deferred unmapping of memory in
> > acpi_ex_system_memory_space_handler() so as to avoid RCU-related
> > performance issues with memory opregions.
>
> ...
>
> > +static bool acpi_os_drop_map_ref(struct acpi_ioremap *map, bool defer)
> >  {
> > -       unsigned long refcount = --map->refcount;
> > +       if (--map->track.refcount)
> > +               return true;
> >
> > -       if (!refcount)
> > -               list_del_rcu(&map->list);
> > -       return refcount;
> > +       list_del_rcu(&map->list);
> > +
>
> > +       if (defer) {
> > +               INIT_LIST_HEAD(&map->track.gc);
> > +               list_add_tail(&map->track.gc, &unused_mappings);
>
> > +               return true;
> > +       }
> > +
> > +       return false;
>
> A nit:
>
> Effectively it returns a value of defer.
>
>   return defer;
>
> >  }

Do you mean that one line of code could be saved?  Yes, it could.

>
> ...
>
> > @@ -416,26 +421,102 @@ void __ref acpi_os_unmap_iomem(void __iomem *virt, acpi_size size)
> >         }
> >
> >         mutex_lock(&acpi_ioremap_lock);
> > +
> >         map = acpi_map_lookup_virt(virt, size);
>
> A nit: should it be somewhere else (I mean in another patch)?

Do you mean the extra empty line?

No, I don't think so, or the code style after this patch would not
look consistent.

> >         if (!map) {
>
> ...
>
> > +       /* Release the unused mappings in the list. */
> > +       while (!list_empty(&list)) {
> > +               struct acpi_ioremap *map;
> > +
> > +               map = list_entry(list.next, struct acpi_ioremap, track.gc);
>
> A nt: if __acpi_os_map_cleanup() (actually acpi_unmap() according to
> the code) has no side effects, can we use list_for_each_entry_safe()
> here?

I actually prefer a do .. while version of this which saves the
initial check (which has been carried out already).

> > +               list_del(&map->track.gc);
> > +               __acpi_os_map_cleanup(map);
> > +       }
> > +}
>
> ...
>
> > @@ -472,16 +552,18 @@ void acpi_os_unmap_generic_address(struct acpi_generic_address *gas)
> >                 return;
> >
> >         mutex_lock(&acpi_ioremap_lock);
> > +
> >         map = acpi_map_lookup(addr, gas->bit_width / 8);
>
> A nit: should it be somewhere else (I mean in another patch)?

Nope.

Thanks!