linux-kernel - Re: [PATCH] drm: Replace drm_modeset_lock/unlock_all with DRM_MODESET_LOCK_ALL

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Message-ID: <20200504115309.GJ10381@phenom.ffwll.local>
Date:   Mon, 4 May 2020 13:53:09 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Michał Orzeł <michalorzel.eng@...il.com>
Cc:     Daniel Vetter <daniel@...ll.ch>, Sean Paul <seanpaul@...omium.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Dave Airlie <airlied@...ux.ie>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH] drm: Replace drm_modeset_lock/unlock_all with
 DRM_MODESET_LOCK_ALL_* helpers

On Fri, May 01, 2020 at 05:49:33PM +0200, Michał Orzeł wrote:
> 
> 
> On 30.04.2020 20:30, Daniel Vetter wrote:
> > On Thu, Apr 30, 2020 at 5:38 PM Sean Paul <seanpaul@...omium.org> wrote:
> >>
> >> On Wed, Apr 29, 2020 at 4:57 AM Jani Nikula <jani.nikula@...ux.intel.com> wrote:
> >>>
> >>> On Tue, 28 Apr 2020, Michal Orzel <michalorzel.eng@...il.com> wrote:
> >>>> As suggested by the TODO list for the kernel DRM subsystem, replace
> >>>> the deprecated functions that take/drop modeset locks with new helpers.
> >>>>
> >>>> Signed-off-by: Michal Orzel <michalorzel.eng@...il.com>
> >>>> ---
> >>>>  drivers/gpu/drm/drm_mode_object.c | 10 ++++++----
> >>>>  1 file changed, 6 insertions(+), 4 deletions(-)
> >>>>
> >>>> diff --git a/drivers/gpu/drm/drm_mode_object.c b/drivers/gpu/drm/drm_mode_object.c
> >>>> index 35c2719..901b078 100644
> >>>> --- a/drivers/gpu/drm/drm_mode_object.c
> >>>> +++ b/drivers/gpu/drm/drm_mode_object.c
> >>>> @@ -402,12 +402,13 @@ int drm_mode_obj_get_properties_ioctl(struct drm_device *dev, void *data,
> >>>>  {
> >>>>       struct drm_mode_obj_get_properties *arg = data;
> >>>>       struct drm_mode_object *obj;
> >>>> +     struct drm_modeset_acquire_ctx ctx;
> >>>>       int ret = 0;
> >>>>
> >>>>       if (!drm_core_check_feature(dev, DRIVER_MODESET))
> >>>>               return -EOPNOTSUPP;
> >>>>
> >>>> -     drm_modeset_lock_all(dev);
> >>>> +     DRM_MODESET_LOCK_ALL_BEGIN(dev, ctx, 0, ret);
> >>>
> >>> I cry a little every time I look at the DRM_MODESET_LOCK_ALL_BEGIN and
> >>> DRM_MODESET_LOCK_ALL_END macros. :(
> >>>
> >>> Currently only six users... but there are ~60 calls to
> >>> drm_modeset_lock_all{,_ctx} that I presume are to be replaced. I wonder
> >>> if this will come back and haunt us.
> >>>
> >>
> >> What's the alternative? Seems like the options without the macros is
> >> to use incorrect scope or have a bunch of retry/backoff cargo-cult
> >> everywhere (and hope the copy source is done correctly).
> > 
> > Yeah Sean & me had a bunch of bikesheds and this is the least worst
> > option we could come up with. You can't make it a function because of
> > the control flow. You don't want to open code this because it's tricky
> > to get right, if all you want is to just grab all locks. But it is
> > magic hidden behind a macro, which occasionally ends up hurting.
> > -Daniel
> So what are we doing with this problem? Should we replace at once approx. 60 calls?

I'm confused by your question - dradual conversion is entirely orthogonal
to what exactly we're converting too. All I added here is that we've
discussed this at length, and the macro is the best thing we've come up
with. I still think it's the best compromise.

Flag-day conversion for over 60 calls doesn't work, no matter what.
-Daniel

> 
> Michal
> > 
> >> Sean
> >>
> >>> BR,
> >>> Jani.
> >>>
> >>>
> >>>>
> >>>>       obj = drm_mode_object_find(dev, file_priv, arg->obj_id, arg->obj_type);
> >>>>       if (!obj) {
> >>>> @@ -427,7 +428,7 @@ int drm_mode_obj_get_properties_ioctl(struct drm_device *dev, void *data,
> >>>>  out_unref:
> >>>>       drm_mode_object_put(obj);
> >>>>  out:
> >>>> -     drm_modeset_unlock_all(dev);
> >>>> +     DRM_MODESET_LOCK_ALL_END(ctx, ret);
> >>>>       return ret;
> >>>>  }
> >>>>
> >>>> @@ -449,12 +450,13 @@ static int set_property_legacy(struct drm_mode_object *obj,
> >>>>  {
> >>>>       struct drm_device *dev = prop->dev;
> >>>>       struct drm_mode_object *ref;
> >>>> +     struct drm_modeset_acquire_ctx ctx;
> >>>>       int ret = -EINVAL;
> >>>>
> >>>>       if (!drm_property_change_valid_get(prop, prop_value, &ref))
> >>>>               return -EINVAL;
> >>>>
> >>>> -     drm_modeset_lock_all(dev);
> >>>> +     DRM_MODESET_LOCK_ALL_BEGIN(dev, ctx, 0, ret);
> >>>>       switch (obj->type) {
> >>>>       case DRM_MODE_OBJECT_CONNECTOR:
> >>>>               ret = drm_connector_set_obj_prop(obj, prop, prop_value);
> >>>> @@ -468,7 +470,7 @@ static int set_property_legacy(struct drm_mode_object *obj,
> >>>>               break;
> >>>>       }
> >>>>       drm_property_change_valid_put(prop, ref);
> >>>> -     drm_modeset_unlock_all(dev);
> >>>> +     DRM_MODESET_LOCK_ALL_END(ctx, ret);
> >>>>
> >>>>       return ret;
> >>>>  }
> >>>
> >>> --
> >>> Jani Nikula, Intel Open Source Graphics Center
> > 
> > 
> > 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch