Re: [PATCH 10/11] drm/i915: use xa_lock/unlock for fpriv->vm_xa lookups

[Maarten Lankhorst <maarten.lankhorst@linux.intel.com>]

Op 13-08-2021 om 22:30 schreef Daniel Vetter:
> We don't need the absolute speed of rcu for this. And
> i915_address_space in general dont need rcu protection anywhere else,
> after we've made gem contexts and engines a lot more immutable.
>
> Note that this semantically reverts
>
> commit aabbe344dc3ca5f7d8263a02608ba6179e8a4499
> Author: Chris Wilson <chris@chris-wilson.co.uk>
> Date:   Fri Aug 30 19:03:25 2019 +0100
>
>     drm/i915: Use RCU for unlocked vm_idr lookup
>
> except we have the conversion from idr to xarray in between.
>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Jon Bloomfield <jon.bloomfield@intel.com>
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
> Cc: Matthew Auld <matthew.auld@intel.com>
> Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
> Cc: Dave Airlie <airlied@redhat.com>
> Cc: Jason Ekstrand <jason@jlekstrand.net>
> ---
>  drivers/gpu/drm/i915/i915_drv.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 005b1cec7007..e37fac8fac0c 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -1881,11 +1881,11 @@ i915_gem_vm_lookup(struct drm_i915_file_private *file_priv, u32 id)
>  {
>  	struct i915_address_space *vm;
>  
> -	rcu_read_lock();
> +	xa_lock(&file_priv->vm_xa);
>  	vm = xa_load(&file_priv->vm_xa, id);
>  	if (vm && !kref_get_unless_zero(&vm->ref))
>  		vm = NULL;
I think this could be a plain i915_vm_get now, kref_get_unless_zero is not guarded by RCU any more.
> -	rcu_read_unlock();
> +	xa_unlock(&file_priv->vm_xa);
>  
>  	return vm;
>  }

Apart from that, all looks good.

With this fix, for patch 2-11:

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>