Re: [PATCH v3 1/2] rust: irq: add support for request_irq()

[Alice Ryhl]

On Tue, Jun 3, 2025 at 11:43 AM Danilo Krummrich <dakr@xxxxxxxxxx> wrote:
>
> On Tue, Jun 03, 2025 at 11:18:40AM +0200, Alice Ryhl wrote:
> > I don't think that helps. If Devres::drop gets to swap is_available
> > before the devm callback performs the swap, then the devm callback is
> > just a no-op and the device still doesn't wait for free_irq() to
> > finish running.
>
> True, this will indeed always be racy. The rule from the C API has always been
> that devm_{remove,release}_action() must not be called if a concurrent unbind
> can't be ruled out. Consequently, the same is true for Revocable::revoke() in
> this case.
>
> I think Devres::drop() shouldn't do anything then and instead we should provide
> Devres::release() and Devres::remove(), which require the &Device<Bound>
> reference the Devres object was created with, in order to prove that there
> can't be a concurrent unbind, just like Devres::access().

What I suggested with the mutex would work if you remove the devm
callback *after* calling free_irq.

    // drop Registration
    mutex_lock();
    free_irq();
    mutex_unlock();
    devm_remove_callback();

    // devm callback
    mutex_lock();
    free_irq();
    mutex_unlock();

Another simpler option is to just not support unregistering the irq
callback except through devm. Then you don't have a registration at
all. Creating the callback can take an irq number and a ForeignOwnable
to put in the void pointer. The devm callback calls free_irq and drops
the ForeignOwnable.

Alice