[PATCH 1/3] dma_resv: prime lockdep annotations

Thomas Hellström (VMware) thomas_os at shipmail.org
Thu Aug 22 06:42:41 UTC 2019


On 8/21/19 9:51 PM, Daniel Vetter wrote:
> 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?

Yes, we do, in a couple of places it seems, and it also appears like TTM 
is doing it according to Christian.

>
> 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.

IMO it should be sufficient to establish this locking order once, but I 
guess dma-buf module init time won't work because we might not have an 
mm structure?

/Thomas

>
> Ideas?
> -Daniel




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