Re: [PATCH] drm/amdgpu: Fix potential race processing vm->freed

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Am 06.02.23 um 19:21 schrieb Rob Clark:
On Mon, Feb 6, 2023 at 8:05 AM Christian König <christian.koenig@xxxxxxx> wrote:
Am 06.02.23 um 16:52 schrieb Rob Clark:
On Mon, Feb 6, 2023 at 2:15 AM Christian König <christian.koenig@xxxxxxx> wrote:
Am 03.02.23 um 19:10 schrieb Rob Clark:
From: Rob Clark <robdclark@xxxxxxxxxxxx>

If userspace calls the AMDGPU_CS ioctl from multiple threads, because
the vm is global to the drm_file, you can end up with multiple threads
racing in amdgpu_vm_clear_freed().  So the freed list should be
protected with the status_lock, similar to other vm lists.
Well this is nonsense. To process the freed list the VM root PD lock
must be held anyway.

If we have a call path where this isn't true then we have a major bug at
a different place here.
I'm not super familiar w/ the amdgpu cs parser stuff, but the only
thing that I'm seeing that protects things is the bo_list_mutex and it
isn't clear to me that this is 1:1 with the vm (it looks like it is
not).
Do you have a backtrace?

Take a look at the reservation object of vm->root.bo. This should always
be locked first before doing *anything* in a CS.

If that isn't the case we have a much worse problem.
In this case, maybe an dma_resv_assert_held() would be a good idea?

We should already have that. Which makes me really wonder what the heck is going on here.

Christian.


BR,
-R

(I cc'd you on the bug report, jfyi)
I unfortunately only get a permission denied when I try to access that one.

Regards,
Christian.

BR,
-R

Regards,
Christian.

Fixes: d38ceaf99ed0 ("drm/amdgpu: add core driver (v4)")
Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
---
    drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c | 33 ++++++++++++++++++++++----
    1 file changed, 29 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
index b9441ab457ea..aeed7bc1512f 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
@@ -1240,10 +1240,19 @@ int amdgpu_vm_clear_freed(struct amdgpu_device *adev,
        struct amdgpu_bo_va_mapping *mapping;
        uint64_t init_pte_value = 0;
        struct dma_fence *f = NULL;
+     struct list_head freed;
        int r;

-     while (!list_empty(&vm->freed)) {
-             mapping = list_first_entry(&vm->freed,
+     /*
+      * Move the contents of the VM's freed list to a local list
+      * that we can iterate without racing against other threads:
+      */
+     spin_lock(&vm->status_lock);
+     list_replace_init(&vm->freed, &freed);
+     spin_unlock(&vm->status_lock);
+
+     while (!list_empty(&freed)) {
+             mapping = list_first_entry(&freed,
                        struct amdgpu_bo_va_mapping, list);
                list_del(&mapping->list);

@@ -1258,6 +1267,15 @@ int amdgpu_vm_clear_freed(struct amdgpu_device *adev,
                amdgpu_vm_free_mapping(adev, vm, mapping, f);
                if (r) {
                        dma_fence_put(f);
+
+                     /*
+                      * Move any unprocessed mappings back to the freed
+                      * list:
+                      */
+                     spin_lock(&vm->status_lock);
+                     list_splice_tail(&freed, &vm->freed);
+                     spin_unlock(&vm->status_lock);
+
                        return r;
                }
        }
@@ -1583,11 +1601,14 @@ int amdgpu_vm_bo_unmap(struct amdgpu_device *adev,
        mapping->bo_va = NULL;
        trace_amdgpu_vm_bo_unmap(bo_va, mapping);

-     if (valid)
+     if (valid) {
+             spin_lock(&vm->status_lock);
                list_add(&mapping->list, &vm->freed);
-     else
+             spin_unlock(&vm->status_lock);
+     } else {
                amdgpu_vm_free_mapping(adev, vm, mapping,
                                       bo_va->last_pt_update);
+     }

        return 0;
    }
@@ -1671,7 +1692,9 @@ int amdgpu_vm_bo_clear_mappings(struct amdgpu_device *adev,
                    tmp->last = eaddr;

                tmp->bo_va = NULL;
+             spin_lock(&vm->status_lock);
                list_add(&tmp->list, &vm->freed);
+             spin_unlock(&vm->status_lock);
                trace_amdgpu_vm_bo_unmap(NULL, tmp);
        }

@@ -1788,7 +1811,9 @@ void amdgpu_vm_bo_del(struct amdgpu_device *adev,
                amdgpu_vm_it_remove(mapping, &vm->va);
                mapping->bo_va = NULL;
                trace_amdgpu_vm_bo_unmap(bo_va, mapping);
+             spin_lock(&vm->status_lock);
                list_add(&mapping->list, &vm->freed);
+             spin_unlock(&vm->status_lock);
        }
        list_for_each_entry_safe(mapping, next, &bo_va->invalids, list) {
                list_del(&mapping->list);




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