linux-kernel - Re: [PATCH 2/6] amd/amdkfd: Ignore return code of dma_fence

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Message-ID: <d46f753e660694ab46c65409a5e43f050b7eb2d9.camel@mailbox.org>
Date: Thu, 27 Nov 2025 10:48:00 +0100
From: Philipp Stanner <phasta@...lbox.org>
To: "Kuehling, Felix" <felix.kuehling@....com>, Philipp Stanner
 <phasta@...nel.org>, Sumit Semwal <sumit.semwal@...aro.org>, Gustavo
 Padovan <gustavo@...ovan.org>, Christian König
 <christian.koenig@....com>, Alex Deucher <alexander.deucher@....com>, David
 Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, Jani Nikula
 <jani.nikula@...ux.intel.com>, Joonas Lahtinen
 <joonas.lahtinen@...ux.intel.com>,  Rodrigo Vivi <rodrigo.vivi@...el.com>,
 Tvrtko Ursulin <tursulin@...ulin.net>, Huang Rui <ray.huang@....com>, 
 Matthew Auld <matthew.auld@...el.com>, Matthew Brost
 <matthew.brost@...el.com>, Maarten Lankhorst
 <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
 Thomas Zimmermann <tzimmermann@...e.de>, Lucas De Marchi
 <lucas.demarchi@...el.com>, Thomas Hellström
 <thomas.hellstrom@...ux.intel.com>
Cc: linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org, 
	amd-gfx@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org, 
	intel-xe@...ts.freedesktop.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 2/6] amd/amdkfd: Ignore return code of dma_fence_signal()

On Wed, 2025-11-26 at 16:24 -0500, Kuehling, Felix wrote:
> 
> On 2025-11-26 08:19, Philipp Stanner wrote:
> > The return code of dma_fence_signal() is not really useful as there is
> > nothing reasonable to do if a fence was already signaled. That return
> > code shall be removed from the kernel.
> > 
> > Ignore dma_fence_signal()'s return code.
> 
> I think this is not correct. Looking at the comment in 
> evict_process_worker, we use the return value to decide a race 
> conditions where multiple threads are trying to signal the eviction 
> fence. Only one of them should schedule the restore work. And the other 
> ones need to increment the reference count to keep evictions balanced.

Thank you for pointing that out. Seems then amdkfd is the only user who
actually relies on the feature. See below

> 
> Regards,
>    Felix
> 
> 
> > 
> > Suggested-by: Christian König <christian.koenig@....com>
> > Signed-off-by: Philipp Stanner <phasta@...nel.org>
> > ---
> >   drivers/gpu/drm/amd/amdkfd/kfd_process.c | 5 ++---
> >   1 file changed, 2 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_process.c b/drivers/gpu/drm/amd/amdkfd/kfd_process.c
> > index ddfe30c13e9d..950fafa4b3c3 100644
> > --- a/drivers/gpu/drm/amd/amdkfd/kfd_process.c
> > +++ b/drivers/gpu/drm/amd/amdkfd/kfd_process.c
> > @@ -1986,7 +1986,6 @@ kfd_process_gpuid_from_node(struct kfd_process *p, struct kfd_node *node,
> >   static int signal_eviction_fence(struct kfd_process *p)
> >   {
> >   	struct dma_fence *ef;
> > -	int ret;
> >   
> >   	rcu_read_lock();
> >   	ef = dma_fence_get_rcu_safe(&p->ef);
> > @@ -1994,10 +1993,10 @@ static int signal_eviction_fence(struct kfd_process *p)
> >   	if (!ef)
> >   		return -EINVAL;
> >   
> > -	ret = dma_fence_signal(ef);
> > +	dma_fence_signal(ef);

The issue now is that dma_fence_signal()'s return code is actually non-
racy, because check + bit-set are protected by lock.

Christian's new spinlock series would add a lock function for fences:
https://lore.kernel.org/dri-devel/20251113145332.16805-5-christian.koenig@amd.com/


So I suppose this should work:

dma_fence_lock_irqsave(ef, flags);
if (dma_fence_test_signaled_flag(ef)) {
	dma_fence_unlock_irqrestore(ef, flags);
	return true;
}
dma_fence_signal_locked(ef);
dma_fence_unlock_irqrestore(ef, flags);

return false;


+ some cosmetic adjustments for the boolean of course.


Would that fly and be reasonable? @Felix, Christian.


P.