On Tue, 03 Oct 2023 14:25:56 +0200
Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx> wrote:
+/**
+ * get_next_vm_bo_from_list() - get the next vm_bo element
+ * @__gpuvm: The GPU VM
+ * @__list_name: The name of the list we're iterating on
+ * @__local_list: A pointer to the local list used to
store
already iterated items
+ * @__prev_vm_bo: The previous element we got from
drm_gpuvm_get_next_cached_vm_bo()
+ *
+ * This helper is here to provide lockless list iteration.
Lockless as in, the
+ * iterator releases the lock immediately after picking
the
first element from
+ * the list, so list insertion deletion can happen
concurrently.
+ *
+ * Elements popped from the original list are kept in a
local
list, so removal
+ * and is_empty checks can still happen while we're
iterating
the list.
+ */
+#define get_next_vm_bo_from_list(__gpuvm, __list_name,
__local_list, __prev_vm_bo) \
+ ({
\
+ struct drm_gpuvm_bo *__vm_bo =
NULL; \
+
\
+ drm_gpuvm_bo_put(__prev_vm_bo);
\
+
\
+ spin_lock(&(__gpuvm)-
__list_name.lock); \
Here we unconditionally take the spinlocks while iterating,
and the
main
point of DRM_GPUVM_RESV_PROTECTED was really to avoid that?
+ if (!(__gpuvm)-
__list_name.local_list) \
+ (__gpuvm)->__list_name.local_list =
__local_list; \
+ else
\
+ WARN_ON((__gpuvm)-
__list_name.local_list
!= __local_list); \
+
\
+ while (!list_empty(&(__gpuvm)-
__list_name.list))
{ \
+ __vm_bo =
list_first_entry(&(__gpuvm)-
__list_name.list, \
+ struct
drm_gpuvm_bo, \
+
list.entry.__list_name); \
+ if (kref_get_unless_zero(&__vm_bo-
kref))
{
And unnecessarily grab a reference in the RESV_PROTECTED
case.
\
+ list_move_tail(&(__vm_bo)-
list.entry.__list_name, \
+
__local_list); \
+ break;
\
+ } else
{ \
+ list_del_init(&(__vm_bo)-
list.entry.__list_name); \
+ __vm_bo =
NULL; \
+ }
\
+ }
\
+ spin_unlock(&(__gpuvm)-
__list_name.lock); \
+
\
+ __vm_bo;
\
+ })
IMHO this lockless list iteration looks very complex and
should be
pretty difficult to maintain while moving forward, also since
it
pulls
the gpuvm_bos off the list, list iteration needs to be
protected by
an
outer lock anyway.
As being partly responsible for this convoluted list iterator,
I must
say I agree with you. There's so many ways this can go wrong if
the
user doesn't call it the right way, or doesn't protect
concurrent
list
iterations with a separate lock (luckily, this is a private
iterator). I
mean, it works, so there's certainly a way to get it right, but
gosh,
this is so far from the simple API I had hoped for.
Also from what I understand from Boris, the extobj
list would typically not need the fine-grained locking; only
the
evict
list?
Right, I'm adding the gpuvm_bo to extobj list in the ioctl
path, when
the GEM and VM resvs are held, and I'm deferring the
drm_gpuvm_bo_put()
call to a work that's not in the dma-signalling path. This
being
said,
I'm still not comfortable with the
gem = drm_gem_object_get(vm_bo->gem);
dma_resv_lock(gem->resv);
drm_gpuvm_bo_put(vm_bo);
dma_resv_unlock(gem->resv);
drm_gem_object_put(gem);
dance that's needed to avoid a UAF when the gpuvm_bo is the
last GEM
owner, not to mention that drm_gpuva_unlink() calls
drm_gpuvm_bo_put()
after making sure the GEM gpuvm_list lock is held, but this
lock
might
differ from the resv lock (custom locking so we can call
gpuvm_unlink() in the dma-signalling path). So we now have
paths
where
drm_gpuvm_bo_put() are called with the resv lock held, and
others
where
they are not, and that only works because we're relying on the
the
fact
those drm_gpuvm_bo_put() calls won't make the refcount drop to
zero,
because the deferred vm_bo_put() work still owns a vm_bo ref.
I'm not sure I follow to 100% here, but in the code snippet above
it's
pretty clear to me that it needs to hold an explicit gem object
reference when calling dma_resv_unlock(gem->resv). Each time you
copy a
referenced pointer (here from vm_bo->gem to gem) you need to up
the
refcount unless you make sure (by locks or other means) that the
source
of the copy has a strong refcount and stays alive, so that's no
weird
action to me. Could possibly add a drm_gpuvm_bo_get_gem() to
access the
gem member (and that also takes a refcount) for driver users to
avoid
the potential pitfall.
Except this is only needed because of the GEM-resv-must-be-held
locking
constraint that was added on vm_bo_put(). I mean, the usual way we
do
object un-referencing is by calling _put() and letting the internal
logic undo things when the refcount drops to zero. If the object
needs
to be removed from some list, it's normally the responsibility of
the
destruction method to lock the list, remove the object and unlock
the
list. Now, we have a refcounted object that's referenced by vm_bo,
and
whose lock needs to be taken when the destruction happens, which
leads
to this weird dance described above, when, in normal situations,
we'd
just call drm_gpuvm_bo_put(vm_bo) and let drm_gpuvm do its thing.
All these tiny details add to the overall complexity of this
common
layer, and to me, that's not any better than the
get_next_vm_bo_from_list() complexity you were complaining
about
(might
be even worth, because this sort of things leak to users).
Having an internal lock partly solves that, in that the locking
of
the
extobj list is now entirely orthogonal to the GEM that's being
removed
from this list, and we can lock/unlock internally without
forcing the
caller to take weird actions to make sure things don't explode.
Don't
get me wrong, I get that this locking overhead is not
acceptable for
Xe, but I feel like we're turning drm_gpuvm into a white
elephant
that
only few people will get right.
I tend to agree, but to me the big complication comes from the
async
(dma signalling path) state updates.
I don't deny updating the VM state from the dma signalling path
adds
some amount of complexity, but the fact we're trying to use
dma_resv
locks for everything, including protection of internal datasets
doesn't
help. Anyway, I think both of us are biased when it comes to
judging
which approach adds the most complexity :P.
Also note that, right now, the only thing I'd like to be able to
update
from the dma signalling path is the VM mapping tree. Everything
else
(drm_gpuva_[un]link(), add/remove extobj), we could do outside this
path:
- for MAP operations, we could call drm_gpuva_link() in the ioctl
path
(we'd just need to initialize the drm_gpuva object)
- for MAP operations, we're already calling drm_gpuvm_bo_obtain()
from
the ioctl path
- for UNMAP operations, we could add the drm_gpuva_unlink() call to
the
VM op cleanup worker
The only problem we'd have is that drm_gpuva_link() needs to be
called
inside drm_gpuvm_ops::sm_step_remap() when a remap with next/prev
!=
NULL occurs, otherwise we lose track of these mappings.
Let's say for example we have a lower level lock for the gem
object's
gpuvm_bo list. Some drivers grab it from the dma fence signalling
path,
other drivers need to access all vm's of a bo to grab their
dma_resv
locks using a WW transaction. There will be problems, although
probably
solveable.
To me, the gpuvm extobj vm_bo list is just an internal list and has
an
internal lock associated. The lock that's protecting the GEM vm_bo
list
is a bit different in that the driver gets to decide when a vm_bo
is
inserted/removed by calling drm_gpuvm_[un]link(), and can easily
make
sure the lock is held when this happens, while the gpuvm internal
lists
are kinda transparently updated (for instance, the first caller of
drm_gpuvm_bo_obtain() adds the vm_bo to the extobj and the last
vm_bo
owner calling drm_gpuvm_bo_put() removes it from this list, which
is
certainly not obvious based on the name of these functions).
If we want to let drivers iterate over the extobj/evict lists, and
assuming they are considered internal lists maintained by the core
and
protected with an internal lock, we should indeed provide iteration
helpers that:
1/ make sure all the necessary external locks are held (VM resv, I
guess)
2/ make sure the internal lock is not held during iteration (the
sort
of snapshot list trick you're using for the evict list in Xe)
Also it seems that if we are to maintain two modes here, for
reasonably clean code we'd need two separate instances of
get_next_bo_from_list().
For the !RESV_PROTECTED case, perhaps one would want to
consider
the
solution used currently in xe, where the VM maintains two
evict
lists.
One protected by a spinlock and one protected by the VM resv.
When
the
VM resv is locked to begin list traversal, the spinlock is
locked
*once*
and the spinlock-protected list is looped over and copied
into the
resv
protected one. For traversal, the resv protected one is
used.
Oh, so you do have the same sort of trick where you move the
entire
list to another list, such that you can let other paths update
the
list
while you're iterating your own snapshot. That's
interesting...
Yes, it's instead of the "evicted" bool suggested here. I thought
the
latter would be simpler. Although that remains to be seen after
all
use-cases are implemented.
But in general I think the concept of copying from a staging list
to
another with different protection rather than traversing the
first list
and unlocking between items is a good way of solving the locking
inversion problem with minimal overhead. We use it also for O(1)
userptr validation.
That's more or less the idea behind get_next_vm_bo_from_list()
except
it's dequeuing one element at a time, instead of moving all items
at
once. Note that, if you allow concurrent removal protected only by
the
spinlock, you still need to take/release this spinlock when
iterating
over elements of this snapshot list, because all the remover needs
to
remove an element is the element itself, and it doesn't care in
which
list it's currently inserted (real or snapshot/staging list), so
you'd
be iterating over a moving target if you don't protect the
iteration
with the spinlock.
