linux-kernel - Re: [PATCH v3 1/2] drm/gpuvm: add deferred vm

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Message-ID: <20251001171346.482142eb@fedora>
Date: Wed, 1 Oct 2025 17:13:46 +0200
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Danilo Krummrich <dakr@...nel.org>, Matthew Brost
 <matthew.brost@...el.com>, Thomas Hellström
 <thomas.hellstrom@...ux.intel.com>, Maarten Lankhorst
 <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
 Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Steven Price <steven.price@....com>,
 Daniel Almeida <daniel.almeida@...labora.com>, Liviu Dudau
 <liviu.dudau@....com>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v3 1/2] drm/gpuvm: add deferred vm_bo cleanup

On Wed, 1 Oct 2025 16:42:35 +0200
Alice Ryhl <aliceryhl@...gle.com> wrote:

> On Wed, Oct 1, 2025 at 4:01 PM Danilo Krummrich <dakr@...nel.org> wrote:
> >
> > On Wed Oct 1, 2025 at 12:41 PM CEST, Alice Ryhl wrote:  
> > > +/*
> > > + * Must be called with GEM mutex held. After releasing GEM mutex,
> > > + * drm_gpuvm_bo_defer_free_unlocked() must be called.
> > > + */
> > > +static void
> > > +drm_gpuvm_bo_defer_free_locked(struct kref *kref)
> > > +{
> > > +     struct drm_gpuvm_bo *vm_bo = container_of(kref, struct drm_gpuvm_bo,
> > > +                                               kref);
> > > +     struct drm_gpuvm *gpuvm = vm_bo->vm;
> > > +
> > > +     if (!drm_gpuvm_resv_protected(gpuvm)) {
> > > +             drm_gpuvm_bo_list_del(vm_bo, extobj, true);
> > > +             drm_gpuvm_bo_list_del(vm_bo, evict, true);
> > > +     }
> > > +
> > > +     list_del(&vm_bo->list.entry.gem);
> > > +}
> > > +
> > > +/*
> > > + * GEM mutex must not be held. Called after drm_gpuvm_bo_defer_free_locked().
> > > + */
> > > +static void
> > > +drm_gpuvm_bo_defer_free_unlocked(struct drm_gpuvm_bo *vm_bo)
> > > +{
> > > +     struct drm_gpuvm *gpuvm = vm_bo->vm;
> > > +
> > > +     llist_add(&vm_bo->list.entry.bo_defer, &gpuvm->bo_defer);
> > > +}
> > > +
> > > +static void
> > > +drm_gpuvm_bo_defer_free(struct kref *kref)
> > > +{
> > > +     struct drm_gpuvm_bo *vm_bo = container_of(kref, struct drm_gpuvm_bo,
> > > +                                               kref);
> > > +
> > > +     mutex_lock(&vm_bo->obj->gpuva.lock);
> > > +     drm_gpuvm_bo_defer_free_locked(kref);
> > > +     mutex_unlock(&vm_bo->obj->gpuva.lock);
> > > +
> > > +     /*
> > > +      * It's important that the GEM stays alive for the duration in which we
> > > +      * hold the mutex, but the instant we add the vm_bo to bo_defer,
> > > +      * another thread might call drm_gpuvm_bo_deferred_cleanup() and put
> > > +      * the GEM. Therefore, to avoid kfreeing a mutex we are holding, we add
> > > +      * the vm_bo to bo_defer *after* releasing the GEM's mutex.
> > > +      */
> > > +     drm_gpuvm_bo_defer_free_unlocked(vm_bo);
> > > +}  
> >
> > So, you're splitting drm_gpuvm_bo_defer_free() into two functions, one doing the
> > work that is required to be called with the gpuva lock held and one that does
> > the work that does not require a lock, which makes perfect sense.
> >
> > However, the naming chosen for the two functions, i.e.
> > drm_gpuvm_bo_defer_free_unlocked() and drm_gpuvm_bo_defer_free_locked() is
> > confusing:
> >
> > What you mean semantically mean is "do part 1 with lock held" and "do part 2
> > without lock held", but the the chosen names suggest that both functions are
> > identical, with the only difference that one takes the lock internally and the
> > other one requires the caller to take the lock.
> >
> > It's probably better to name them after what they do and not what they're part
> > of. If you prefer the latter, that's fine with me too, but please choose a name
> > that makes this circumstance obvious.  
> 
> Fair point. Do you have naming suggestions? Otherwise I can name them
> drm_gpuvm_bo_defer_free_part1() and drm_gpuvm_bo_defer_free_part2().
> :)

drm_gpuvm_bo_free_deferral_extract_locked() and
drm_gpuvm_bo_free_deferral_enqueue()? Definitely not short names though.