Re: [PATCH v4 10/15] drm/shmem-helper: Take reservation lock instead of drm_gem_shmem locks

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

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.

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.

Regards,
Christian.

>
>> There are actually very few drivers in kernel that use dma_buf_vmap()
>> [1], so perhaps it's not really a big deal to first try to define the
>> locking and pinning convention for the dma-bufs? At least for
>> dma_buf_vmap()? Let me try to do this.
>>
>> [1] https://elixir.bootlin.com/linux/v5.18-rc6/C/ident/dma_buf_vmap
> Yeah looking through the code there's largely two classes of drivers that
> need vmap:
>
> - display drivers that need to do cpu upload (usb, spi, i2c displays).
>    Those generally set up the vmap at import time or when creating the
>    drm_framebuffer object (e.g. see
>    drm_gem_cma_prime_import_sg_table_vmap()), because that's really the
>    only place where you can safely do that without running into locking
>    inversion issues sooner or later
>
> - lots of other drivers (and shmem helpers) seem to do dma_buf_vmap just
>    because they can, but only actually ever use vmap on native objects,
>    never on imported objects. Or at least I think so.
>
> So maybe another approach here:
>
> 1. In general drivers which need a vmap need to set that up at dma_buf
> import time - the same way we pin the buffers at import time for
> non-dynamic importers because that's the only place where across all
> drivers it's ok to just take dma_resv_lock.
>
> 2. We remove the "just because we can" dma_buf_vmap support from
> helpers/drivers - the paths all already can cope with NULL since
> dma_buf_vmap can fail. vmap will only work on native objects, not imported
> ones.
>
> 3. If there is any driver using shmem helpers that absolutely needs vmap
> to also work on imported it needs a special import function (like cma
> helpers) which sets up the vmap at import time.
>
> So since this is all very tricky ... what did I miss this time around?
>
>> I envision that the extra dma_resv_locks for dma-bufs potentially may
>> create unnecessary bottlenecks for some drivers if locking isn't really
>> necessary by a specific driver, so drivers will need to keep this in
>> mind. On the other hand, I don't think that any of the today's drivers
>> will notice the additional resv locks in practice.
> Nah I don't think the extra locking will ever create a bottleneck,
> especially not for vmap. Generally vmap is a fallback or at least cpu
> operation, so at that point you're already going very slow.
> -Daniel

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