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 5ED8E197548; Fri, 6 Dec 2024 07:50:05 +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=1733471405; cv=none; b=jrho4k/UAKBOjRBM3DUpHOklOOrSIMpbvlxSziPVZpvtoFG4XcRG9GVHCQNEMQFZSOIEPQgYsY2VwzA1g1LL+Hvedsy+KqEmvdaKwWmoAgxtGxwNYu6v/DjAM424QIjGK3ubth2OROZ9kQMSahy7W0U6jqHTzwh31xGDmqGMl/A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1733471405; c=relaxed/simple; bh=5qohqrW2C5S36QEHyyoNx6PWg6lLbKY+njcv5MoaFPo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=H0beJHksH3k+j/WrttgPwq5nMeS80w5hffwZuTUOllnRBTK/On7Vbj9F/KXba7l/rtIp4ACebSXrCXveghjGPsZgvYdvr5VljbAMjfWtyr/HaDGR/0pUMvFbNv7mZVa0GEig0AwNoIpeyMhaEB8XyBnGjOdHcAJmFIDLNdWkgYo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=FIQc8BGt; 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="FIQc8BGt" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 49127C4CED1; Fri, 6 Dec 2024 07:50:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1733471404; bh=5qohqrW2C5S36QEHyyoNx6PWg6lLbKY+njcv5MoaFPo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=FIQc8BGtQAhiH4JJOAmbVnJS7eOBDYeUIs8VcbSY1Bh7RC0FewpPoRboXrDUlWtVd IXL1YVvUUjTTrVaIFE1KqUyYQRgX8SU8ZRIBGTj0N0ENBaT+e25Z8APG8gpGPptzIF aF/pz2xoW3PC+6QmZE5mJ+70KWi7KjoM06NhBk1DsQ+TWf+NXZS9Y7dyYnVgWaJhsc pYPlVM0TjKc8//qkpn7d9UN5n+zOr3kHXi45hDfr6EJ27LWnAyj1afZSES9htadg1h jrsKaXaQG1B6vdfIIgkdVCJVsTx+vLHSPkDcF2NCWA8zwKq5im38s6HNVP1EknFN2O Qb7OJ6NwmE1lg== Date: Fri, 6 Dec 2024 07:49:59 +0000 From: Lee Jones To: Greg KH Cc: linux-kernel@vger.kernel.org, arnd@arndb.de, ojeda@kernel.org, alex.gaynor@gmail.com, boqun.feng@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: <20241206074959.GL8882@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> <2024120616-endanger-strangely-62b4@gregkh> Precedence: bulk X-Mailing-List: rust-for-linux@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: <2024120616-endanger-strangely-62b4@gregkh> On Fri, 06 Dec 2024, Greg KH 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() > > once it goes out of scope. > > > > // SAFETY: Instances of `Device` are always reference-counted. > > unsafe impl crate::types::AlwaysRefCounted for Device { > > fn inc_ref(&self) { > > // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero. > > unsafe { bindings::get_device(self.as_raw()) }; > > } > > > > unsafe fn dec_ref(obj: ptr::NonNull) { > > // SAFETY: The safety requirements guarantee that the refcount is non-zero. > > unsafe { bindings::put_device(obj.cast().as_ptr()) } > > } > > Ick, really? So as_ref() implicitly calles inc_ref() and dec_ref()? > Ah, ok, in digging into AlwaysRefCounted I now see that seems to be > true. > > So great, this is a reference counted object, so what's preventing it > from now being used in dev_info()? We're having this conversation in stereo at this point. TL;DR, we can't call MiscDeviceRegistration::device() after .init YET. The longer version of this can be seen in the cover-letter thread. -- Lee Jones [李琼斯]