linux-kernel - Re: [PATCH 1/4] drm/gpuvm: take GEM lock inside drm_gpuvm_bo_obtain

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Message-ID: <aS1l9Mm9smUzAJ3A@google.com>
Date: Mon, 1 Dec 2025 09:55:00 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Boris Brezillon <boris.brezillon@...labora.com>
Cc: Danilo Krummrich <dakr@...nel.org>, Daniel Almeida <daniel.almeida@...labora.com>, 
	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>, Liviu Dudau <liviu.dudau@....com>, 
	Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Frank Binns <frank.binns@...tec.com>, Matt Coster <matt.coster@...tec.com>, 
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	Rodrigo Vivi <rodrigo.vivi@...el.com>, Sumit Semwal <sumit.semwal@...aro.org>, 
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Subject: Re: [PATCH 1/4] drm/gpuvm: take GEM lock inside drm_gpuvm_bo_obtain_prealloc()

On Fri, Nov 28, 2025 at 03:24:03PM +0100, Boris Brezillon wrote:
> On Fri, 28 Nov 2025 14:14:15 +0000
> Alice Ryhl <aliceryhl@...gle.com> wrote:
> 
> > When calling drm_gpuvm_bo_obtain_prealloc() and using immediate mode,
> > this may result in a call to ops->vm_bo_free(vm_bo) while holding the
> > GEMs gpuva mutex. This is a problem if ops->vm_bo_free(vm_bo) performs
> > any operations that are not safe in the fence signalling critical path,
> > and it turns out that Panthor (the only current user of the method)
> > calls drm_gem_shmem_unpin() which takes a resv lock internally.
> > 
> > This constitutes both a violation of signalling safety and lock
> > inversion. To fix this, we modify the method to internally take the GEMs
> > gpuva mutex so that the mutex can be unlocked before freeing the
> > preallocated vm_bo.
> > 
> > Note that this modification introduces a requirement that the driver
> > uses immediate mode to call drm_gpuvm_bo_obtain_prealloc() as it would
> > otherwise take the wrong lock.
> > 
> > Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> 
> Reviewed-by: Boris Brezillon <boris.brezillon@...labora.com>
> 
> Should we add a Fixes tag?

Yeah, let's add:

Fixes: 63e919a31625 ("panthor: use drm_gpuva_unlink_defer()")

Alice