Re: [PATCH v13 01/10] drm/shmem-helper: Switch to reservation lock

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

On 3/15/23 16:46, Dmitry Osipenko wrote:
> On 3/14/23 05:26, Dmitry Osipenko wrote:
>> @@ -633,7 +605,10 @@ int drm_gem_shmem_mmap(struct drm_gem_shmem_object *shmem, struct vm_area_struct
>>  		return ret;
>>  	}
>>  
>> +	dma_resv_lock(shmem->base.resv, NULL);
>>  	ret = drm_gem_shmem_get_pages(shmem);
>> +	dma_resv_unlock(shmem->base.resv);
> 
> Intel CI reported locking problem [1] here. It actually was also
> reported for v12, but I missed that report because of the other noisy
> reports.
> 
> [1]
> https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_114671v2/shard-snb5/igt@prime_vgem@sync@rcs0.html
> 
> The test does the following:
> 
> 1. creates vgem
> 2. export it do dmabuf
> 3. mmaps dmabuf
> 
> There is an obvious deadlock there. The DRM code assumes that mmap() is
> unlocked, while for dma-buf it's locked. I'll see how to fix it for v14.
> 

Christian, there is a deadlock problem in drm_gem_shmem_mmap() once we
move drm-shmem to use resv lock. The current dma-buf locking policy
claims that importer holds the lock for mmap(), but DRM code assumes
that obj->mmap() handles the locking itself and then the same
obj->mmap() code path is used by both dma-buf DRM and a usual DRM object
paths. Hence importer -> dma_buf_mmap_internal()[takes the lock] ->
exporter -> drm_gem_shmem_mmap()[takes the lock] deadlocks.

I was looking at how to fix it and to me the best option is to change
the dma-buf locking policy, making exporter responsible for handling the
resv lock. Changing DRM code mmap lockings might be possible too [moving
locking to drm_gem_mmap_obj()], but will be very messy and doesn't feel
intuitive.

Want to get yours thoughts on this before sending out the dma-buf mmap()
policy-change patch. Does the new mmap() locking policy sound good to
you? Thanks!

-- 
Best regards,
Dmitry