Re: [PATCH 09/12] of: overlay: avoid race condition between applying multiple overlays

[Rob Herring]

On Wed, Oct 04, 2017 at 08:29:59PM -0700, Frank Rowand wrote:
> On 10/04/17 08:19, Rob Herring wrote:
> > On Mon, Oct 2, 2017 at 10:53 PM,  <frowand.list@xxxxxxxxx> wrote:
> >> From: Frank Rowand <frank.rowand@xxxxxxxx>
> >>
> >> The process of applying an overlay consists of:
> >>   - unflatten an overlay FDT (flattened device tree) into an
> >>     EDT (expanded device tree)
> >>   - fixup the phandle values in the overlay EDT to fit in a
> >>     range above the phandle values in the live device tree
> >>   - create the overlay changeset to reflect the contents of
> >>     the overlay EDT
> >>   - apply the overlay changeset, to modify the live device tree,
> >>     potentially changing the maximum phandle value in the live
> >>     device tree
> >>
> >> There is currently no protection against two overlay applies
> >> concurrently determining what range of phandle values are in use
> >> in the live device tree, and subsequently changing that range.
> >> Add a mutex to prevent multiple overlay applies from occurring
> >> simultaneously.
> >>
> >> Ignoring 2 checkpatch warnings: Prefer using '"%s...", __func__'
> >> so that the WARN() string will be more easily grepped.
> >>
> >> Signed-off-by: Frank Rowand <frank.rowand@xxxxxxxx>
> >> ---
> >>  drivers/gpu/drm/tilcdc/tilcdc_slave_compat.c |  7 +++++++
> >>  drivers/of/overlay.c                         | 22 ++++++++++++++++++++++
> >>  drivers/of/unittest.c                        | 21 +++++++++++++++++++++
> >>  include/linux/of.h                           | 19 +++++++++++++++++++
> >>  4 files changed, 69 insertions(+)
> >>
> >> diff --git a/drivers/gpu/drm/tilcdc/tilcdc_slave_compat.c b/drivers/gpu/drm/tilcdc/tilcdc_slave_compat.c
> >> index 7a7be0515bfd..c99f7924b1c6 100644
> >> --- a/drivers/gpu/drm/tilcdc/tilcdc_slave_compat.c
> >> +++ b/drivers/gpu/drm/tilcdc/tilcdc_slave_compat.c
> >> @@ -221,6 +221,11 @@ static void __init tilcdc_convert_slave_node(void)
> >>                 goto out;
> >>         }
> >>
> >> +       /*
> >> +        * protect from of_resolve_phandles() through of_overlay_apply()
> >> +        */
> >> +       of_overlay_mutex_lock();
> >> +
> > 
> > We can't be relying on callers to get the locking right...
> 
> Agreed.
> 
> 
> > 
> >>         overlay = tilcdc_get_overlay(&kft);
> >>         if (!overlay)
> >>                 goto out;
> >> @@ -256,6 +261,8 @@ static void __init tilcdc_convert_slave_node(void)
> >>                 pr_info("%s: ti,tilcdc,slave node successfully converted\n",
> >>                         __func__);
> >>  out:
> >> +       of_overlay_mutex_unlock();
> >> +
> >>         kfree_table_free(&kft);
> >>         of_node_put(i2c);
> >>         of_node_put(slave);
> >> diff --git a/drivers/of/overlay.c b/drivers/of/overlay.c
> >> index a0d3222febdc..4ed372af6ce7 100644
> >> --- a/drivers/of/overlay.c
> >> +++ b/drivers/of/overlay.c
> >> @@ -71,6 +71,28 @@ static int build_changeset_next_level(struct overlay_changeset *ovcs,
> >>                 const struct device_node *overlay_node,
> >>                 bool is_symbols_node);
> >>
> >> +/*
> >> + * of_resolve_phandles() finds the largest phandle in the live tree.
> >> + * of_overlay_apply() may add a larger phandle to the live tree.
> >> + * Do not allow race between two overlays being applied simultaneously:
> >> + *    mutex_lock(&of_overlay_phandle_mutex)
> >> + *    of_resolve_phandles()
> >> + *    of_overlay_apply()
> >> + *    mutex_unlock(&of_overlay_phandle_mutex)
> > 
> > Why do these need to be separate functions? I think I mentioned it
> > before, but essentially overlay_data_add() should be part of the
> > overlay API. We may need to allow for callers to do each step, but
> > generally I think the interface should just be "apply this fdt blob".
> 
> Yes, that is where I want to end up.

So, is that not doable now? To put it another way, why does 
of_resolve_phandles need to be a separate call? Seems like an internal 
detail of how you apply an overlay to me.

Rob