[PATCH 1/3] dma_resv: prime lockdep annotations

Wed Aug 21 19:51:10 UTC 2019

On Wed, Aug 21, 2019 at 08:27:59PM +0200, Thomas Hellström (VMware) wrote:
> On 8/21/19 8:11 PM, Daniel Vetter wrote:
> > On Wed, Aug 21, 2019 at 7:06 PM Thomas Hellström (VMware)
> > <thomas_os at shipmail.org> wrote:
> > > On 8/21/19 6:34 PM, Daniel Vetter wrote:
> > > > On Wed, Aug 21, 2019 at 05:54:27PM +0200, Thomas Hellström (VMware) wrote:
> > > > > On 8/20/19 4:53 PM, Daniel Vetter wrote:
> > > > > > Full audit of everyone:
> > > > > > 
> > > > > > - i915, radeon, amdgpu should be clean per their maintainers.
> > > > > > 
> > > > > > - vram helpers should be fine, they don't do command submission, so
> > > > > >      really no business holding struct_mutex while doing copy_*_user. But
> > > > > >      I haven't checked them all.
> > > > > > 
> > > > > > - panfrost seems to dma_resv_lock only in panfrost_job_push, which
> > > > > >      looks clean.
> > > > > > 
> > > > > > - v3d holds dma_resv locks in the tail of its v3d_submit_cl_ioctl(),
> > > > > >      copying from/to userspace happens all in v3d_lookup_bos which is
> > > > > >      outside of the critical section.
> > > > > > 
> > > > > > - vmwgfx has a bunch of ioctls that do their own copy_*_user:
> > > > > >      - vmw_execbuf_process: First this does some copies in
> > > > > >        vmw_execbuf_cmdbuf() and also in the vmw_execbuf_process() itself.
> > > > > >        Then comes the usual ttm reserve/validate sequence, then actual
> > > > > >        submission/fencing, then unreserving, and finally some more
> > > > > >        copy_to_user in vmw_execbuf_copy_fence_user. Glossing over tons of
> > > > > >        details, but looks all safe.
> > > > > >      - vmw_fence_event_ioctl: No ttm_reserve/dma_resv_lock anywhere to be
> > > > > >        seen, seems to only create a fence and copy it out.
> > > > > >      - a pile of smaller ioctl in vmwgfx_ioctl.c, no reservations to be
> > > > > >        found there.
> > > > > >      Summary: vmwgfx seems to be fine too.
> > > > > > 
> > > > > > - virtio: There's virtio_gpu_execbuffer_ioctl, which does all the
> > > > > >      copying from userspace before even looking up objects through their
> > > > > >      handles, so safe. Plus the getparam/getcaps ioctl, also both safe.
> > > > > > 
> > > > > > - qxl only has qxl_execbuffer_ioctl, which calls into
> > > > > >      qxl_process_single_command. There's a lovely comment before the
> > > > > >      __copy_from_user_inatomic that the slowpath should be copied from
> > > > > >      i915, but I guess that never happened. Try not to be unlucky and get
> > > > > >      your CS data evicted between when it's written and the kernel tries
> > > > > >      to read it. The only other copy_from_user is for relocs, but those
> > > > > >      are done before qxl_release_reserve_list(), which seems to be the
> > > > > >      only thing reserving buffers (in the ttm/dma_resv sense) in that
> > > > > >      code. So looks safe.
> > > > > > 
> > > > > > - A debugfs file in nouveau_debugfs_pstate_set() and the usif ioctl in
> > > > > >      usif_ioctl() look safe. nouveau_gem_ioctl_pushbuf() otoh breaks this
> > > > > >      everywhere and needs to be fixed up.
> > > > > > 
> > > > > > Cc: Alex Deucher <alexander.deucher at amd.com>
> > > > > > Cc: Christian König <christian.koenig at amd.com>
> > > > > > Cc: Chris Wilson <chris at chris-wilson.co.uk>
> > > > > > Cc: Thomas Zimmermann <tzimmermann at suse.de>
> > > > > > Cc: Rob Herring <robh at kernel.org>
> > > > > > Cc: Tomeu Vizoso <tomeu.vizoso at collabora.com>
> > > > > > Cc: Eric Anholt <eric at anholt.net>
> > > > > > Cc: Dave Airlie <airlied at redhat.com>
> > > > > > Cc: Gerd Hoffmann <kraxel at redhat.com>
> > > > > > Cc: Ben Skeggs <bskeggs at redhat.com>
> > > > > > Cc: "VMware Graphics" <linux-graphics-maintainer at vmware.com>
> > > > > > Cc: Thomas Hellstrom <thellstrom at vmware.com>
> > > > > > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > > > > > ---
> > > > > >     drivers/dma-buf/dma-resv.c | 12 ++++++++++++
> > > > > >     1 file changed, 12 insertions(+)
> > > > > > 
> > > > > > diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
> > > > > > index 42a8f3f11681..3edca10d3faf 100644
> > > > > > --- a/drivers/dma-buf/dma-resv.c
> > > > > > +++ b/drivers/dma-buf/dma-resv.c
> > > > > > @@ -34,6 +34,7 @@
> > > > > >     #include <linux/dma-resv.h>
> > > > > >     #include <linux/export.h>
> > > > > > +#include <linux/sched/mm.h>
> > > > > >     /**
> > > > > >      * DOC: Reservation Object Overview
> > > > > > @@ -107,6 +108,17 @@ void dma_resv_init(struct dma_resv *obj)
> > > > > >                      &reservation_seqcount_class);
> > > > > >      RCU_INIT_POINTER(obj->fence, NULL);
> > > > > >      RCU_INIT_POINTER(obj->fence_excl, NULL);
> > > > > > +
> > > > > > +   if (IS_ENABLED(CONFIG_LOCKDEP)) {
> > > > > > +           if (current->mm)
> > > > > > +                   down_read(&current->mm->mmap_sem);
> > > > > > +           ww_mutex_lock(&obj->lock, NULL);
> > > > > > +           fs_reclaim_acquire(GFP_KERNEL);
> > > > > > +           fs_reclaim_release(GFP_KERNEL);
> > > > > > +           ww_mutex_unlock(&obj->lock);
> > > > > > +           if (current->mm)
> > > > > > +                   up_read(&current->mm->mmap_sem);
> > > > > > +   }
> > > > > >     }
> > > > > >     EXPORT_SYMBOL(dma_resv_init);
> > > > > I assume if this would have been easily done and maintainable using only
> > > > > lockdep annotation instead of actually acquiring the locks, that would have
> > > > > been done?
> > > > There's might_lock(), plus a pile of macros, but they don't map obviuosly,
> > > > so pretty good chances I accidentally end up with the wrong type of
> > > > annotation. Easier to just take the locks quickly, and stuff that all into
> > > > a lockdep-only section to avoid overhead.
> > > > 
> > > > > Otherwise LGTM.
> > > > > 
> > > > > Reviewed-by: Thomas Hellström <thellstrom at vmware.com>
> > > > > 
> > > > > Will test this and let you know if it trips on vmwgfx, but it really
> > > > > shouldn't.
> > > > Thanks, Daniel
> > > One thing that strikes me is that this puts restrictions on where you
> > > can actually initialize a dma_resv, even if locking orders are otherwise
> > > obeyed. But that might not be a big problem.
> > Hm yeah ... the trouble is a need a non-kthread thread so that I have
> > a current->mm. Otherwise I'd have put it into some init section with a
> > temp dma_buf. And I kinda don't want to create a fake ->mm just for
> > lockdep priming. I don't expect this to be a real problem in practice,
> > since before you've called dma_resv_init the reservation lock doesn't
> > exist, so you can't hold it. And you've probably just allocated it, so
> > fs_reclaim is going to be fine. And if you allocate dma_resv objects
> > from your fault handlers I have questions anyway :-)
> 
> Coming to think of it, I think vmwgfx sometimes create bos with other bo's
> reservation lock held. I guess that would trip both the mmap_sem check the
> ww_mutex check?

If you do that, yes we're busted. Do you do that?

I guess needs a new idea for where to put this ... while making sure
everyone gets it. So some evil trick like putting it in drm_open() won't
work, since I also want to make sure everyone else using dma-buf follows
these rules.

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