Re: [PATCH] drm/panthor: Fix the "done_fence is initialized" detection logic

[Nicolas Frattaroli <nicolas.frattaroli@collabora.com>]

On Monday, 9 March 2026 11:30:53 Central European Standard Time Boris Brezillon wrote:
> After commit 541c8f2468b9 ("dma-buf: detach fence ops on signal v3"),
> dma_fence::ops == NULL can't be used to check if the fence is initialized
> or not. We could turn this into an "is_signaled() || ops == NULL" test,
> but that's fragile, since it's still subject to dma_fence internal
> changes. So let's have the "is_initialized" state encoded directly in
> the pointer through the lowest bit which is guaranteed to be unused
> because of the dma_fence alignment constraint.
> 
> Cc: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
> Cc: Philipp Stanner <phasta@kernel.org>
> Cc: Christian König <christian.koenig@amd.com>
> Reported-by: Steven Price <steven.price@arm.com>
> Reported-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
> Fixes: 541c8f2468b9 ("dma-buf: detach fence ops on signal v3")
> Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
> ---
>  drivers/gpu/drm/panthor/panthor_sched.c | 69 ++++++++++++++++++-------
>  1 file changed, 50 insertions(+), 19 deletions(-)

Tested-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>

This fixes a refcount underflow for me when I do something that spams
a bunch of fatal exceptions and then terminate the userspace
application that produces them.