From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E465C3D6B; Fri, 6 Dec 2024 08:31:22 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1733473883; cv=none; b=LX+TgstHfngiXWOis82sPL4sQxBQhA5TxrgHvDVhm+RjYVDyfPpWfvB+ThxDnt0McL32xAuWpt+KiY2MrnSJ6+BhtSR0rjpzeLqvp7mZbYyeR7FBQEyMQ5ehFtJBD1MfNmiGf5Y0pzxXcpL+GRdv/OjK+k+lFu7Rwqdtf2WIWqQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1733473883; c=relaxed/simple; bh=wwVvhOgw3TqhMZRruQ7sCYqRQDH1bBCjL08Ym4VjgvU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=tJpUQq9QEje/2vVx0XZsvDT/4TCAso8Guov+2BGISAAuwDeRGwVbwWv5xMw+yAJLDaUzkoo2Mhn3CQtv7TawpTzvBKuvwtGqSDTZFwIeBt6A+BnwFZSZLL09SNcZwFdfOSuySrZ5pqbFr+wFHPuYX2fRMIg0ig8Eq+qoxr/99Po= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=L3kljBk2; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="L3kljBk2" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D5341C4CEDD; Fri, 6 Dec 2024 08:31:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1733473882; bh=wwVvhOgw3TqhMZRruQ7sCYqRQDH1bBCjL08Ym4VjgvU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=L3kljBk22ib53sm2gMc8/CWKnxk3bkb6G8LQP9kGEgokpKPz0YPjeebupaNgcB7/g mTsFT5/EURcVIoGg0fhx6tq5kLjCN+2HZYbd96JDu3QYljh1090fXk29BxGrt4fl7c ZKZGT9E+YJ7nZLruVxOjjjJe/i5jENoPWHFp8QMqikVhwvA2CfpcWp2BLufI8FQbd+ j4AVz6D67PZ4wB67RBHq4SlkHXFdJ3CnjMRTcBovnuORvfdEvIx7UXQ+WDqJTnc0WY tLinVmkwsgna7/5ozkUv5TDqG+IFjk0NAuYoaJlr9MBM+dGFqpvfu3mMGbiJrGYr49 nj7Q60MWq0FPw== Date: Fri, 6 Dec 2024 08:31:17 +0000 From: Lee Jones To: Boqun Feng Cc: Greg KH , linux-kernel@vger.kernel.org, arnd@arndb.de, ojeda@kernel.org, alex.gaynor@gmail.com, gary@garyguo.net, bjorn3_gh@protonmail.com, benno.lossin@proton.me, a.hindborg@kernel.org, aliceryhl@google.com, tmgross@umich.edu, rust-for-linux@vger.kernel.org Subject: Re: [PATCH v3 1/5] rust: miscdevice: Provide accessor to pull out miscdevice::this_device Message-ID: <20241206083117.GO8882@google.com> References: <20241205162531.1883859-1-lee@kernel.org> <20241205162531.1883859-2-lee@kernel.org> <2024120648-finch-shrubbery-c6f5@gregkh> <20241206071646.GE8882@google.com> <20241206073309.GG8882@google.com> <20241206080751.GN8882@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Fri, 06 Dec 2024, Boqun Feng wrote: > On Fri, Dec 06, 2024 at 08:07:51AM +0000, Lee Jones wrote: > > On Fri, 06 Dec 2024, Boqun Feng wrote: > > > > > On Fri, Dec 06, 2024 at 07:33:09AM +0000, Lee Jones wrote: > > > > On Fri, 06 Dec 2024, Lee Jones wrote: > > > > > > > > > On Fri, 06 Dec 2024, Greg KH wrote: > > > > > > > > > > > On Thu, Dec 05, 2024 at 04:25:18PM +0000, Lee Jones wrote: > > > > > > > There are situations where a pointer to a `struct device` will become > > > > > > > necessary (e.g. for calling into dev_*() functions). This accessor > > > > > > > allows callers to pull this out from the `struct miscdevice`. > > > > > > > > > > > > > > Signed-off-by: Lee Jones > > > > > > > --- > > > > > > > rust/kernel/miscdevice.rs | 9 +++++++++ > > > > > > > 1 file changed, 9 insertions(+) > > > > > > > > > > > > > > diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs > > > > > > > index 7e2a79b3ae26..55340f316006 100644 > > > > > > > --- a/rust/kernel/miscdevice.rs > > > > > > > +++ b/rust/kernel/miscdevice.rs > > > > > > > @@ -10,11 +10,13 @@ > > > > > > > > > > > > > > use crate::{ > > > > > > > bindings, > > > > > > > + device::Device, > > > > > > > error::{to_result, Error, Result, VTABLE_DEFAULT_ERROR}, > > > > > > > prelude::*, > > > > > > > str::CStr, > > > > > > > types::{ForeignOwnable, Opaque}, > > > > > > > }; > > > > > > > + > > > > > > > use core::{ > > > > > > > ffi::{c_int, c_long, c_uint, c_ulong}, > > > > > > > marker::PhantomData, > > > > > > > @@ -84,6 +86,13 @@ pub fn register(opts: MiscDeviceOptions) -> impl PinInit { > > > > > > > pub fn as_raw(&self) -> *mut bindings::miscdevice { > > > > > > > self.inner.get() > > > > > > > } > > > > > > > + > > > > > > > + /// Returns a pointer to the current Device > > > > > > > + pub fn device(&self) -> &Device { > > > > > > > + // SAFETY: This is only accessible after a successful register() which always > > > > > > > + // initialises this_device with a valid device. > > > > > > > + unsafe { Device::as_ref((*self.as_raw()).this_device) } > > > > > > > > > > > > A "raw" pointer that you can do something with without incrementing the > > > > > > reference count of it? Oh wait, no, it's the rust device structure. > > > > > > If so, why isn't this calling the get_device() interface instead? That > > > > > > way it's properly incremented and decremented when it "leaves the scope" > > > > > > right? > > > > > > > > > > > > Or am I missing something here as to why that wouldn't work and this is > > > > > > the only way to get access to the 'struct device' of this miscdevice? > > > > > > > > > > Fair point. I'll speak to Alice. > > > > > > > > Alice isn't available yet, so I may be talking out of turn at this > > > > point, but I just found this is the Device documentation: > > > > > > > > /// A `Device` instance represents a valid `struct device` created by the C portion of the kernel. > > > > /// > > > > /// Instances of this type are always reference-counted, that is, a call to `get_device` ensures > > > > /// that the allocation remains valid at least until the matching call to `put_device`. > > > > > > > > And: > > > > > > > > // SAFETY: Instances of `Device` are always reference-counted. > > > > > > > > Ready for some analysis from this beginner? > > > > > > > > Since this impl for Device is AlwaysRefCounted, when any references are > > > > taken i.e. in the Device::as_ref line above, inc_ref() is implicitly > > > > called to increase the refcount. The same will be true of dec_ref() > > > > > > No, inc_ref() is not called implicitly in Device::as_ref(). > > > > > > The thing that might "keep" the original `miscdevice::Device` alive is > > > the lifetime, since the returned `device::Device` reference has the > > > same life at the input parameter `miscdevice::Device` reference (i.e. > > > `&self`), so the returned reference cannot outlive `&self`. That means > > > if compilers find `&self` go out of scope while the returned reference > > > be still alive, it will report an error. > > > > Okay, so is there something I need to do to ensure we increase the > > refcount? Does inc_ref() need calling manually? > > > > When you convert a `&Device` into a `ARef`, Device::inc_ref() > will be called. You can do that with: > > ARef::from(Device::as_ref((*self.as_raw()).this_device)) > > You will also need to change the return type. And when an `ARef` > goes out of scope, dec_ref() will be called. I have been reliably assured by Alice that we don't need to refcount here. > I had an old patch for a bit document on this part: > > https://lore.kernel.org/rust-for-linux/20240710032447.2161189-1-boqun.feng@gmail.com/ > > maybe I should send a re-spin. Very nice! Yeah, it would be a shame for all that work to go to waste. -- Lee Jones [李琼斯]