Re: [PATCH 1/3] dma_resv: prime lockdep annotations

[Daniel Vetter <daniel@ffwll.ch>]

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@amd.com>
> > Cc: Christian König <christian.koenig@amd.com>
> > Cc: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Thomas Zimmermann <tzimmermann@suse.de>
> > Cc: Rob Herring <robh@kernel.org>
> > Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
> > Cc: Eric Anholt <eric@anholt.net>
> > Cc: Dave Airlie <airlied@redhat.com>
> > Cc: Gerd Hoffmann <kraxel@redhat.com>
> > Cc: Ben Skeggs <bskeggs@redhat.com>
> > Cc: "VMware Graphics" <linux-graphics-maintainer@vmware.com>
> > Cc: Thomas Hellstrom <thellstrom@vmware.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter@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@vmware.com>
> 
> Will test this and let you know if it trips on vmwgfx, but it really
> shouldn't.

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