[PATCH 1/2] dma-buf: add dma_fence_unwrap

[Daniel Vetter]

On Mon, Mar 28, 2022 at 12:28:31PM +0200, Christian König wrote:
> Hi Ville & Daniel,
> 
> Am 25.03.22 um 16:28 schrieb Christian König:
> > Am 25.03.22 um 16:25 schrieb Ville Syrjälä:
> > > On Fri, Mar 11, 2022 at 12:02:43PM +0100, Christian König wrote:
> > > > Add a general purpose helper to deep dive into
> > > > dma_fence_chain/dma_fence_array
> > > > structures and iterate over all the fences in them.
> > > > 
> > > > This is useful when we need to flatten out all fences in those
> > > > structures.
> > > > 
> > > > Signed-off-by: Christian König <christian.koenig at amd.com>
> > > One of the dma-buf patches took down Intel CI. Looks like every
> > > machine oopses in some sync_file thing now:
> > > <1>[  260.470008] BUG: kernel NULL pointer dereference, address:
> > > 0000000000000010
> > > <1>[  260.470020] #PF: supervisor read access in kernel mode
> > > <1>[  260.470025] #PF: error_code(0x0000) - not-present page
> > > <6>[  260.470030] PGD 0 P4D 0
> > > <4>[  260.470035] Oops: 0000 [#1] PREEMPT SMP NOPTI
> > > <4>[  260.470040] CPU: 0 PID: 5306 Comm: core_hotunplug Not tainted
> > > 5.17.0-CI-CI_DRM_11405+ #1
> > > <4>[  260.470049] Hardware name: Intel Corporation Jasper Lake
> > > Client Platform/Jasperlake DDR4 SODIMM RVP, BIOS
> > > JSLSFWI1.R00.2385.D02.2010160831 10/16/2020
> > > <4>[  260.470058] RIP: 0010:dma_fence_array_first+0x19/0x20
> 
> I've looked into this and the root cause seems to be that the new code
> doesn't handle dma_fence_arrays with zero elements.
> 
> That is rather easy to fix, but a dma_fence_array with zero number of
> elements is most likely a bug because under the wrong circumstances it can
> create a dma_fence instance which will never signal.
> 
> I've send out a patch on Frinday ([PATCH] dma-buf: WIP dma_fence_array_first
> fix) which avoids the crash and prints a warning if anybody tries to create
> a dma_fence_array with zero length.
> 
> Can you test this?

It's in drm-tip now (in the fixup branch) so drm-tip results should have
the result soonish:

https://intel-gfx-ci.01.org/tree/drm-tip/index.html?

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch