Re: [RFC PATCH 0/2] Attempt to avoid dma-fence-[chain|array] lockdep splats

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

Am 30.11.21 um 13:19 schrieb Thomas Hellström:
> Introducing more usage of dma-fence-chain and dma-fence-array in the
> i915 driver we start to hit lockdep splats due to the recursive fence
> locking in the dma-fence-chain and dma-fence-array containers.
> This is a humble suggestion to try to establish a dma-fence locking order
> (patch 1) and to avoid excessive recursive locking in these containers
> (patch 2)

Well completely NAK to this.

This splats are intentional notes that something in the driver code is 
wrong (or we messed up the chain and array containers somehow).

Those two containers are intentionally crafted in a way which allows to 
avoid any dependency between their spinlocks. So as long as you 
correctly use them you should never see a splat.

Please provide the lockdep splat so that we can analyze the problem.

Thanks,
Christian.

>
> Thomas Hellström (2):
>    dma-fence: Avoid establishing a locking order between fence classes
>    dma-fence: Avoid excessive recursive fence locking from
>      enable_signaling() callbacks
>
>   drivers/dma-buf/dma-fence-array.c | 23 +++++++--
>   drivers/dma-buf/dma-fence-chain.c | 12 ++++-
>   drivers/dma-buf/dma-fence.c       | 79 +++++++++++++++++++++----------
>   include/linux/dma-fence.h         |  4 ++
>   4 files changed, 89 insertions(+), 29 deletions(-)
>
> Cc: linaro-mm-sig@lists.linaro.org
> Cc: dri-devel@lists.freedesktop.org
> Cc: Christian König <christian.koenig@amd.com>
>