linux-kernel - Re: [PATCH 3/6] drm/gem: use new ww_mutex_(un)lock_for_each macros

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Date:   Sat, 15 Jun 2019 15:56:25 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Christian König <christian.koenig@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Lucas Stach <l.stach@...gutronix.de>,
        Russell King <linux+etnaviv@...linux.org.uk>,
        Christian Gmeiner <christian.gmeiner@...il.com>,
        Qiang Yu <yuq825@...il.com>, "Anholt, Eric" <eric@...olt.net>,
        Thomas Hellstrom <thellstrom@...are.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        The etnaviv authors <etnaviv@...ts.freedesktop.org>,
        lima@...ts.freedesktop.org
Subject: Re: [PATCH 3/6] drm/gem: use new ww_mutex_(un)lock_for_each macros

On Fri, Jun 14, 2019 at 10:30 PM Daniel Vetter <daniel@...ll.ch> wrote:
>
> On Fri, Jun 14, 2019 at 08:51:11PM +0200, Christian König wrote:
> > Am 14.06.19 um 20:24 schrieb Daniel Vetter:
> > >
> > > On Fri, Jun 14, 2019 at 8:10 PM Christian König <ckoenig.leichtzumerken@...il.com> wrote:
> > > > [SNIP]
> > > > WW_MUTEX_LOCK_BEGIN()
> > > >
> > > > lock(lru_lock);
> > > >
> > > > while (bo = list_first(lru)) {
> > > >   if (kref_get_unless_zero(bo)) {
> > > >           unlock(lru_lock);
> > > >           WW_MUTEX_LOCK(bo->ww_mutex);
> > > >           lock(lru_lock);
> > > >   } else {
> > > >           /* bo is getting freed, steal it from the freeing process
> > > >            * or just ignore */
> > > >   }
> > > > }
> > > > unlock(lru_lock)
> > > >
> > > > WW_MUTEX_LOCK_END;
> >
> > Ah, now I know what you mean. And NO, that approach doesn't work.
> >
> > See for the correct ww_mutex dance we need to use the iterator multiple
> > times.
> >
> > Once to give us the BOs which needs to be locked and another time to give us
> > the BOs which needs to be unlocked in case of a contention.
> >
> > That won't work with the approach you suggest here.
>
> A right, drat.
>
> Maybe give up on the idea to make this work for ww_mutex in general, and
> just for drm_gem_buffer_object? I'm just about to send out a patch series
> which makes sure that a lot more drivers set gem_bo.resv correctly (it
> will alias with ttm_bo.resv for ttm drivers ofc). Then we could add a
> list_head to gem_bo (won't really matter, but not something we can do with
> ww_mutex really), so that the unlock walking doesn't need to reuse the
> same iterator. That should work I think ...
>
> Also, it would almost cover everything you want to do. For ttm we'd need
> to make ttm_bo a subclass of gem_bo (and maybe not initialize that
> embedded gem_bo for vmwgfx and shadow bo and driver internal stuff).
>
> Just some ideas, since copypasting the ww_mutex dance into all drivers is
> indeed not great.

Even better we don't need to force everyone to use drm_gem_object, the
hard work is already done with the reservation_object. We could add a
list_head there for unwinding, and then the locking helpers would look
a lot cleaner and simpler imo. reservation_unlock_all() would even be
a real function! And if we do this then I think we should also have a
reservation_acquire_ctx, to store the list_head and maybe anything
else.

Plus all the code you want to touch is dealing with
reservation_object, so that's all covered. And it mirros quite a bit
what we've done with struct drm_modeset_lock, to wrap ww_mutex is
something easier to deal with for kms.
-Daniel

> -Daniel
>
> >
> > Regards,
> > Christian.
> >
> > >
> > >
> > > Also I think if we allow this we could perhaps use this to implement the
> > > modeset macros too.
> > > -Daniel
> > >
> > >
> > >
> > >
> > > > > This is kinda what we went with for modeset locks with
> > > > > DRM_MODESET_LOCK_ALL_BEGIN/END, you can grab more locks in between the
> > > > > pair at least. But it's a lot more limited use-cases, maybe too fragile an
> > > > > idea for ww_mutex in full generality.
> > > > >
> > > > > Not going to type this out because too much w/e mode here already, but I
> > > > > can give it a stab next week.
> > > > > -Daniel
> > > > _______________________________________________
> > > > dri-devel mailing list
> > > > dri-devel@...ts.freedesktop.org
> > > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> > >
> > >
> >
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel@...ts.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

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