[Intel-xe] [PATCH] drm/xe/bo: don't hold dma-resv lock over drm_gem_handle_create

[Matthew Auld]

On Mon, 9 Oct 2023 at 10:01, Matthew Auld <matthew.auld at intel.com> wrote:
>
> This seems to create a locking inversion with object_name_lock. The lock
> is held by drm_prime_fd_to_handle when calling our xe_gem_prime_import
> hook, which might eventually go on to grab the dma-resv lock during the
> attach. However we also have the opposite locking order in
> xe_gem_create_ioctl which is holding the dma-resv lock when calling
> drm_gem_handle_create, which wants to eventually grab object_name_lock:
>
> -> #1 (reservation_ww_class_mutex){+.+.}-{3:3}:
> <4> [635.739288]        lock_acquire+0x169/0x3d0
> <4> [635.739294]        __ww_mutex_lock.constprop.0+0x164/0x1e60
> <4> [635.739300]        ww_mutex_lock_interruptible+0x42/0x1a0
> <4> [635.739305]        drm_gem_shmem_pin+0x4b/0x140 [drm_shmem_helper]
> <4> [635.739317]        dma_buf_dynamic_attach+0x101/0x430
> <4> [635.739323]        xe_gem_prime_import+0xcc/0x2e0 [xe]
> <4> [635.739499]        drm_prime_fd_to_handle_ioctl+0x184/0x2e0 [drm]
> <4> [635.739594]        drm_ioctl_kernel+0x16f/0x250 [drm]
> <4> [635.739693]        drm_ioctl+0x35e/0x620 [drm]
> <4> [635.739789]        __x64_sys_ioctl+0xb7/0xf0
> <4> [635.739794]        do_syscall_64+0x3c/0x90
> <4> [635.739799]        entry_SYSCALL_64_after_hwframe+0x6e/0xd8
> <4> [635.739805]
> -> #0 (&dev->object_name_lock){+.+.}-{3:3}:
> <4> [635.739813]        check_prev_add+0x1ba/0x14a0
> <4> [635.739818]        __lock_acquire+0x203e/0x2ff0
> <4> [635.739823]        lock_acquire+0x169/0x3d0
> <4> [635.739827]        __mutex_lock+0x124/0x1310
> <4> [635.739832]        drm_gem_handle_create+0x32/0x50 [drm]
> <4> [635.739927]        xe_gem_create_ioctl+0x1d3/0x550 [xe]
> <4> [635.740102]        drm_ioctl_kernel+0x16f/0x250 [drm]
> <4> [635.740197]        drm_ioctl+0x35e/0x620 [drm]
> <4> [635.740293]        __x64_sys_ioctl+0xb7/0xf0
> <4> [635.740297]        do_syscall_64+0x3c/0x90
> <4> [635.740302]        entry_SYSCALL_64_after_hwframe+0x6e/0xd8
> <4> [635.740307]
>
> It looks like it should be safe to simply drop the dma-resv lock prior
> to publishing the object when calling drm_gem_handle_create.
>
> Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/743
> Signed-off-by: Matthew Auld <matthew.auld at intel.com>
> Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>

Any takers for this one? This will start triggering in BAT once we merge:
https://patchwork.freedesktop.org/series/124667/

Since that has a new BAT test which triggers the external dma-import
path via VGEM.

> ---
>  drivers/gpu/drm/xe/xe_bo.c | 20 ++++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
> index 61789c0e88fb..ff116d3a8861 100644
> --- a/drivers/gpu/drm/xe/xe_bo.c
> +++ b/drivers/gpu/drm/xe/xe_bo.c
> @@ -1815,14 +1815,16 @@ int xe_gem_create_ioctl(struct drm_device *dev, void *data,
>                 if (XE_IOCTL_DBG(xe, !vm))
>                         return -ENOENT;
>                 err = xe_vm_lock(vm, true);
> -               if (err) {
> -                       xe_vm_put(vm);
> -                       return err;
> -               }
> +               if (err)
> +                       goto out_vm;
>         }
>
>         bo = xe_bo_create(xe, NULL, vm, args->size, ttm_bo_type_device,
>                           bo_flags);
> +
> +       if (vm)
> +               xe_vm_unlock(vm);
> +
>         if (IS_ERR(bo)) {
>                 err = PTR_ERR(bo);
>                 goto out_vm;
> @@ -1836,15 +1838,17 @@ int xe_gem_create_ioctl(struct drm_device *dev, void *data,
>         goto out_put;
>
>  out_bulk:
> -       if (vm && !xe_vm_in_fault_mode(vm))
> +       if (vm && !xe_vm_in_fault_mode(vm)) {
> +               xe_vm_lock(vm, false);
>                 __xe_bo_unset_bulk_move(bo);
> +               xe_vm_unlock(vm);
> +       }
>  out_put:
>         xe_bo_put(bo);
>  out_vm:
> -       if (vm) {
> -               xe_vm_unlock(vm);
> +       if (vm)
>                 xe_vm_put(vm);
> -       }
> +
>         return err;
>  }
>
> --
> 2.41.0
>