drm/amdkfd: Fix lock dependency warning

drm/amdkfd: Fix lock dependency warning

======================================================
WARNING: possible circular locking dependency detected
6.5.0-kfd-fkuehlin #276 Not tainted
------------------------------------------------------
kworker/8:2/2676 is trying to acquire lock:
ffff9435aae95c88 ((work_completion)(&svm_bo->eviction_work)){+.+.}-{0:0}, at: __flush_work+0x52/0x550

but task is already holding lock:
ffff9435cd8e1720 (&svms->lock){+.+.}-{3:3}, at: svm_range_deferred_list_work+0xe8/0x340 [amdgpu]

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #2 (&svms->lock){+.+.}-{3:3}:
       __mutex_lock+0x97/0xd30
       kfd_ioctl_alloc_memory_of_gpu+0x6d/0x3c0 [amdgpu]
       kfd_ioctl+0x1b2/0x5d0 [amdgpu]
       __x64_sys_ioctl+0x86/0xc0
       do_syscall_64+0x39/0x80
       entry_SYSCALL_64_after_hwframe+0x63/0xcd

-> #1 (&mm->mmap_lock){++++}-{3:3}:
       down_read+0x42/0x160
       svm_range_evict_svm_bo_worker+0x8b/0x340 [amdgpu]
       process_one_work+0x27a/0x540
       worker_thread+0x53/0x3e0
       kthread+0xeb/0x120
       ret_from_fork+0x31/0x50
       ret_from_fork_asm+0x11/0x20

-> #0 ((work_completion)(&svm_bo->eviction_work)){+.+.}-{0:0}:
       __lock_acquire+0x1426/0x2200
       lock_acquire+0xc1/0x2b0
       __flush_work+0x80/0x550
       __cancel_work_timer+0x109/0x190
       svm_range_bo_release+0xdc/0x1c0 [amdgpu]
       svm_range_free+0x175/0x180 [amdgpu]
       svm_range_deferred_list_work+0x15d/0x340 [amdgpu]
       process_one_work+0x27a/0x540
       worker_thread+0x53/0x3e0
       kthread+0xeb/0x120
       ret_from_fork+0x31/0x50
       ret_from_fork_asm+0x11/0x20

other info that might help us debug this:

Chain exists of:
  (work_completion)(&svm_bo->eviction_work) --> &mm->mmap_lock --> &svms->lock

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&svms->lock);
                               lock(&mm->mmap_lock);
                               lock(&svms->lock);
  lock((work_completion)(&svm_bo->eviction_work));

I believe this cannot really lead to a deadlock in practice, because
svm_range_evict_svm_bo_worker only takes the mmap_read_lock if the BO
refcount is non-0. That means it's impossible that svm_range_bo_release
is running concurrently. However, there is no good way to annotate this.

To avoid the problem, take a BO reference in
svm_range_schedule_evict_svm_bo instead of in the worker. That way it's
impossible for a BO to get freed while eviction work is pending and the
cancel_work_sync call in svm_range_bo_release can be eliminated.

v2: Use svm_bo_ref_unless_zero and explained why that's safe. Also
removed redundant checks that are already done in
amdkfd_fence_enable_signaling.

Signed-off-by: Felix Kuehling <felix.kuehling@amd.com>
Reviewed-by: Philip Yang <philip.yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>