Re: [PATCH v1 4/6] dma-buf: Acquire wait-wound context on attachment

["Christian König" <christian.koenig@amd.com>]

Am 19.07.22 um 22:05 schrieb Dmitry Osipenko:
> On 7/15/22 09:59, Dmitry Osipenko wrote:
>> On 7/15/22 09:50, Christian König wrote:
>>> Am 15.07.22 um 02:52 schrieb Dmitry Osipenko:
>>>> Intel i915 GPU driver uses wait-wound mutex to lock multiple GEMs on the
>>>> attachment to the i915 dma-buf. In order to let all drivers utilize
>>>> shared
>>>> wait-wound context during attachment in a general way, make dma-buf
>>>> core to
>>>> acquire the ww context internally for the attachment operation and update
>>>> i915 driver to use the importer's ww context instead of the internal one.
>>>>
>>>>   From now on all dma-buf exporters shall use the importer's ww context
>>>> for
>>>> the attachment operation.
>>>>
>>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
>>>> ---
>>>>    drivers/dma-buf/dma-buf.c                     |  8 +++++-
>>>>    drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c    |  2 +-
>>>>    .../gpu/drm/i915/gem/i915_gem_execbuffer.c    |  2 +-
>>>>    drivers/gpu/drm/i915/gem/i915_gem_object.h    |  6 ++---
>>>>    drivers/gpu/drm/i915/i915_gem_evict.c         |  2 +-
>>>>    drivers/gpu/drm/i915/i915_gem_ww.c            | 26 +++++++++++++++----
>>>>    drivers/gpu/drm/i915/i915_gem_ww.h            | 15 +++++++++--
>>>>    7 files changed, 47 insertions(+), 14 deletions(-)
>>>>
>>>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>>>> index 0ee588276534..37545ecb845a 100644
>>>> --- a/drivers/dma-buf/dma-buf.c
>>>> +++ b/drivers/dma-buf/dma-buf.c
>>>> @@ -807,6 +807,8 @@ static struct sg_table * __map_dma_buf(struct
>>>> dma_buf_attachment *attach,
>>>>     * Optionally this calls &dma_buf_ops.attach to allow
>>>> device-specific attach
>>>>     * functionality.
>>>>     *
>>>> + * Exporters shall use ww_ctx acquired by this function.
>>>> + *
>>>>     * Returns:
>>>>     *
>>>>     * A pointer to newly created &dma_buf_attachment on success, or a
>>>> negative
>>>> @@ -822,6 +824,7 @@ dma_buf_dynamic_attach_unlocked(struct dma_buf
>>>> *dmabuf, struct device *dev,
>>>>                    void *importer_priv)
>>>>    {
>>>>        struct dma_buf_attachment *attach;
>>>> +    struct ww_acquire_ctx ww_ctx;
>>>>        int ret;
>>>>          if (WARN_ON(!dmabuf || !dev))
>>>> @@ -841,7 +844,8 @@ dma_buf_dynamic_attach_unlocked(struct dma_buf
>>>> *dmabuf, struct device *dev,
>>>>        attach->importer_ops = importer_ops;
>>>>        attach->importer_priv = importer_priv;
>>>>    -    dma_resv_lock(dmabuf->resv, NULL);
>>>> +    ww_acquire_init(&ww_ctx, &reservation_ww_class);
>>>> +    dma_resv_lock(dmabuf->resv, &ww_ctx);
>>> That won't work like this. The core property of a WW context is that you
>>> need to unwind all the locks and re-quire them with the contended one
>>> first.
>>>
>>> When you statically lock the imported one here you can't do that any more.
>> You're right. I felt that something is missing here, but couldn't
>> notice. I'll think more about this and enable
>> CONFIG_DEBUG_WW_MUTEX_SLOWPATH. Thank you!
>>
> Christian, do you think we could make an excuse for the attach()
> callback and make the exporter responsible for taking the resv lock? It
> will be inconsistent with the rest of the callbacks, where importer
> takes the lock, but it will be the simplest and least invasive solution.
> It's very messy to do a cross-driver ww locking, I don't think it's the
> right approach.

So to summarize the following calls will require that the caller hold 
the resv lock:
1. dma_buf_pin()/dma_buf_unpin()
2. dma_buf_map_attachment()/dma_buf_unmap_attachment()
3. dma_buf_vmap()/dma_buf_vunmap()
4. dma_buf_move_notify()

The following calls require that caller does not held the resv lock:
1. dma_buf_attach()/dma_buf_dynamic_attach()/dma_buf_detach()
2. dma_buf_export()/dma_buf_fd()
3. dma_buf_get()/dma_buf_put()
4. dma_buf_begin_cpu_access()/dma_buf_end_cpu_access()

If that's correct than that would work for me as well, but we should 
probably document this.

Or let me ask the other way around: What calls exactly do you need to 
change to solve your original issue? That was vmap/vunmap, wasn't it? If 
yes then let's concentrate on those for the moment.

Regards,
Christian.