Messages in this thread | | | Date | Wed, 20 Jul 2022 10:29:42 +0200 | Subject | Re: [PATCH v1 4/6] dma-buf: Acquire wait-wound context on attachment | From | Christian König <> |
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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.
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