linux-kernel - Re: [RFC PATCH 2/4] rust: sync: Add dma

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Message-ID: <20260210123432.588a20f5@fedora>
Date: Tue, 10 Feb 2026 12:34:32 +0100
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: "Christian König" <christian.koenig@....com>, Philipp
 Stanner <phasta@...lbox.org>, phasta@...nel.org, Danilo Krummrich
 <dakr@...nel.org>, David Airlie <airlied@...il.com>, Simona Vetter
 <simona@...ll.ch>, Gary Guo <gary@...yguo.net>, Benno Lossin
 <lossin@...nel.org>, Daniel Almeida <daniel.almeida@...labora.com>, Joel
 Fernandes <joelagnelf@...dia.com>, linux-kernel@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, rust-for-linux@...r.kernel.org
Subject: Re: [RFC PATCH 2/4] rust: sync: Add dma_fence abstractions

On Tue, 10 Feb 2026 10:15:04 +0000
Alice Ryhl <aliceryhl@...gle.com> wrote:

> impl MustBeSignalled<'_> {
>     /// Drivers generally should not use this one.
>     fn i_promise_it_will_be_signalled(self) -> WillBeSignalled { ... }
> 
>     /// One way to ensure the fence has been signalled is to signal it.
>     fn signal_fence(self) -> WillBeSignalled {
>         self.fence.signal();
>         self.i_promise_it_will_be_signalled()
>     }
> 
>     /// Another way to ensure the fence will be signalled is to spawn a
>     /// workqueue item that promises to signal it.
>     fn transfer_to_wq(
>         self,
>         wq: &Workqueue,
>         item: impl DmaFenceWorkItem,
>     ) -> WillBeSignalled {
>         // briefly obtain the lock class of the wq to indicate to
>         // lockdep that the signalling path "blocks" on arbitrary jobs
>         // from this wq completing
>         bindings::lock_acquire(&wq->key);
>         bindings::lock_release(&wq->key);

Sorry, I'm still trying to connect the dots here. I get that the intent
is to ensure the pseudo-lock ordering is always:

   -> dma_fence_lockdep_map
      -> wq->lockdep_map

but how can this order be the same in the WorkItem execution path? My
interpretation of process_one_work() makes me think we'll end up with

  -> wq->lockdep_map
    -> work->run()
       -> WorkItem::run()
          -> dma_fence_lockdep_map
             -> DmaFenceSignalingWorkItem::run()
                ...

Am I missing something? Is there a way you can insert the
dma_fence_lockdep_map acquisition before the wq->lockdep_map one in the
execution path?

> 
>         // enqueue the job
>         wq.enqueue(item, wq);
> 
>         // The signature of DmaFenceWorkItem::run() promises to arrange
>         // for it to be signalled.
>         self.i_promise_it_will_be_signalled()
>     }
> }