On 5/11/22 18:29, Daniel Vetter wrote: > On Wed, May 11, 2022 at 06:14:00PM +0300, Dmitry Osipenko wrote: >> On 5/11/22 17:24, Christian König wrote: >>> Am 11.05.22 um 15:00 schrieb Daniel Vetter: >>>> On Tue, May 10, 2022 at 04:39:53PM +0300, Dmitry Osipenko wrote: >>>>> [SNIP] >>>>> Since vmapping implies implicit pinning, we can't use a separate lock in >>>>> drm_gem_shmem_vmap() because we need to protect the >>>>> drm_gem_shmem_get_pages(), which is invoked by drm_gem_shmem_vmap() to >>>>> pin the pages and requires the dma_resv_lock to be locked. >>>>> >>>>> Hence the problem is: >>>>> >>>>> 1. If dma-buf importer holds the dma_resv_lock and invokes >>>>> dma_buf_vmap() -> drm_gem_shmem_vmap(), then drm_gem_shmem_vmap() shall >>>>> not take the dma_resv_lock. >>>>> >>>>> 2. Since dma-buf locking convention isn't specified, we can't assume >>>>> that dma-buf importer holds the dma_resv_lock around dma_buf_vmap(). >>>>> >>>>> The possible solutions are: >>>>> >>>>> 1. Specify the dma_resv_lock convention for dma-bufs and make all >>>>> drivers to follow it. >>>>> >>>>> 2. Make only DRM drivers to hold dma_resv_lock around dma_buf_vmap(). >>>>> Other non-DRM drivers will get the lockdep warning. >>>>> >>>>> 3. Make drm_gem_shmem_vmap() to take the dma_resv_lock and get deadlock >>>>> if dma-buf importer holds the lock. >>>>> >>>>> ... >>>> Yeah this is all very annoying. >>> Ah, yes that topic again :) >>> >>> I think we could relatively easily fix that by just defining and >>> enforcing that the dma_resv_lock must have be taken by the caller when >>> dma_buf_vmap() is called. >>> >>> A two step approach should work: >>> 1. Move the call to dma_resv_lock() into the dma_buf_vmap() function and >>> remove all lock taking from the vmap callback implementations. >>> 2. Move the call to dma_resv_lock() into the callers of dma_buf_vmap() >>> and enforce that the function is called with the lock held. >> I've doubts about the need to move out the dma_resv_lock() into the >> callers of dma_buf_vmap().. >> >> I looked through all the dma_buf_vmap() users and neither of them >> interacts with dma_resv_lock() at all, i.e. nobody takes the lock >> in/outside of dma_buf_vmap(). Hence it's easy and more practical to make >> dma_buf_mmap/vmap() to take the dma_resv_lock by themselves. > i915_gem_dmabuf_vmap -> i915_gem_object_pin_map_unlocked -> > i915_gem_object_lock -> dma_resv_lock > > And all the ttm drivers should work similarly. So there's definitely > drivers which grab dma_resv_lock from their vmap callback. Grr.. I'll take another look. >> It's unclear to me which driver may ever want to do the mapping under >> the dma_resv_lock. But if we will ever have such a driver that will need >> to map imported buffer under dma_resv_lock, then we could always add the >> dma_buf_vmap_locked() variant of the function. In this case the locking >> rule will sound like this: >> >> "All dma-buf importers are responsible for holding the dma-reservation >> lock around the dmabuf->ops->mmap/vmap() calls." Are you okay with this rule? >>> It shouldn't be that hard to clean up. The last time I looked into it my >>> main problem was that we didn't had any easy unit test for it. >> Do we have any tests for dma-bufs at all? It's unclear to me what you >> are going to test in regards to the reservation locks, could you please >> clarify? > Unfortunately not really :-/ Only way really is to grab a driver which > needs vmap (those are mostly display drivers) on an imported buffer, and > see what happens. > > 2nd best is liberally sprinkling lockdep annotations all over the place > and throwing it at intel ci (not sure amd ci is accessible to the public) > and then hoping that's good enough. Stuff like might_lock and > dma_resv_assert_held. Alright -- Best regards, Dmitry
