From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-244107.protonmail.ch (mail-244107.protonmail.ch [109.224.244.107]) (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 624E925B67D; Sat, 6 Sep 2025 04:26:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=109.224.244.107 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757132771; cv=none; b=IZ4y+dL1gWIL7+Ztzi56Q8j5hxKDkvwcHl2O/p5FBsESdxGNDw1E03p89ELXYCpBhVhFQaZWALZTejoBIhMxc/2Y+kZFDXUuuHgpTvNZXN9F9Y8s38nUS+KHDq+l2mW+HL7f1gosY5XcvO/VFzD1UorN+cX+IIVbTEde6fwDcec= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757132771; c=relaxed/simple; bh=HRzzQLL7HgyN3IET1oFCb+1tsnaXUdqKgOdnVrDvTkM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=K/x5iMd5shvlkpUmbJz+6aAhLoaDyYlMUx+0RsEjXkVyjVyJmQj0/Q01LsIosTrCxoIZgT0+Xf46FUJLlvi++B/p9m59Bz1LavDN/AOyHIxeH8hvpG1cggDjCpiCX5IJKBtOPtUv5G5zXs+jx9DInNySRBwsUrN3flhQmRt8Wi8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=weathered-steel.dev; spf=pass smtp.mailfrom=weathered-steel.dev; dkim=pass (2048-bit key) header.d=weathered-steel.dev header.i=@weathered-steel.dev header.b=oO5mzYU2; arc=none smtp.client-ip=109.224.244.107 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=weathered-steel.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=weathered-steel.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=weathered-steel.dev header.i=@weathered-steel.dev header.b="oO5mzYU2" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=weathered-steel.dev; s=protonmail3; t=1757132766; x=1757391966; bh=MTSO5mLqb6QtLE1IH2HBBR5v+hWR8KIYiUIKItIkK8U=; h=Date:From:To:Cc:Subject:Message-ID:References:In-Reply-To:From:To: Cc:Date:Subject:Reply-To:Feedback-ID:Message-ID:BIMI-Selector; b=oO5mzYU2xFG81rh9MTmSqbrcja7AgiV5WmiIpAsPPnKbfM/2VL5i2SZNs7td6gyJy qABTkSEgtO/kriO/uhUmYm0jOzQOATJe4oLrGMXzNV8QN2HPiPKtUZf7Il3buwa33u vN3n3nx6sX79iu168sUzOjDTOTldzmxbWGqko14giP2pF11onCqmrGeXz4lBHGVHJr ci+EPvVqA2gjyWvyK7J175HabbbdEwQF5kC6xfuU9DhWo8DixFzJuK8TfXYgY41q/V Gyz3FPReb8eTj1Zx0ZuQvdswXlxIX8qqgUScxXSi+9swosLOpO+1r+49GvOPaqffVz PUKpS5ZRQv+7A== X-Pm-Submission-Id: 4cJgBh4Rtsz2ScDD Date: Sat, 6 Sep 2025 04:26:01 +0000 From: Elle Rhumsaa To: Boqun Feng Cc: rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, lkmm@lists.linux.dev, Will Deacon , Peter Zijlstra , Mark Rutland , Ingo Molnar , Thomas Gleixner , "Paul E. McKenney" , stern@rowland.harvard.edu, Miguel Ojeda , alex.gaynor@gmail.com, Gary Guo , =?iso-8859-1?Q?Bj=F6rn?= Roy Baron , Benno Lossin , Alice Ryhl , Trevor Gross , Danilo Krummrich , Andreas Hindborg , Alexandre Courbot Subject: Re: [PATCH 12/14] rust: convert `Arc` to use `Refcount` Message-ID: References: <20250905044141.77868-1-boqun.feng@gmail.com> <20250905044141.77868-13-boqun.feng@gmail.com> 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=us-ascii Content-Disposition: inline In-Reply-To: <20250905044141.77868-13-boqun.feng@gmail.com> On Thu, Sep 04, 2025 at 09:41:39PM -0700, Boqun Feng wrote: > From: Gary Guo > > With `Refcount` type created, `Arc` can use `Refcount` instead of > calling into FFI directly. > > Signed-off-by: Gary Guo > Reviewed-by: Alexandre Courbot > Reviewed-by: Benno Lossin > Signed-off-by: Boqun Feng > Link: https://lore.kernel.org/r/20250723233312.3304339-4-gary@kernel.org > --- > rust/kernel/sync/arc.rs | 45 +++++++++++++---------------------------- > 1 file changed, 14 insertions(+), 31 deletions(-) > > diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs > index 4ee155b43b2d..9298993ea7d8 100644 > --- a/rust/kernel/sync/arc.rs > +++ b/rust/kernel/sync/arc.rs > @@ -8,7 +8,7 @@ > //! threads. > //! > //! It is different from the standard library's [`Arc`] in a few ways: > -//! 1. It is backed by the kernel's `refcount_t` type. > +//! 1. It is backed by the kernel's [`Refcount`] type. > //! 2. It does not support weak references, which allows it to be half the size. > //! 3. It saturates the reference count instead of aborting when it goes over a threshold. > //! 4. It does not provide a `get_mut` method, so the ref counted object is pinned. > @@ -18,11 +18,11 @@ > > use crate::{ > alloc::{AllocError, Flags, KBox}, > - bindings, > ffi::c_void, > init::InPlaceInit, > + sync::Refcount, > try_init, > - types::{ForeignOwnable, Opaque}, > + types::ForeignOwnable, > }; > use core::{ > alloc::Layout, > @@ -145,7 +145,7 @@ pub struct Arc { > #[pin_data] > #[repr(C)] > struct ArcInner { > - refcount: Opaque, > + refcount: Refcount, > data: T, > } > > @@ -157,7 +157,7 @@ impl ArcInner { > /// `ptr` must have been returned by a previous call to [`Arc::into_raw`], and the `Arc` must > /// not yet have been destroyed. > unsafe fn container_of(ptr: *const T) -> NonNull> { > - let refcount_layout = Layout::new::(); > + let refcount_layout = Layout::new::(); > // SAFETY: The caller guarantees that the pointer is valid. > let val_layout = Layout::for_value(unsafe { &*ptr }); > // SAFETY: We're computing the layout of a real struct that existed when compiling this > @@ -229,8 +229,7 @@ impl Arc { > pub fn new(contents: T, flags: Flags) -> Result { > // INVARIANT: The refcount is initialised to a non-zero value. > let value = ArcInner { > - // SAFETY: There are no safety requirements for this FFI call. > - refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }), > + refcount: Refcount::new(1), > data: contents, > }; > > @@ -348,18 +347,13 @@ pub fn into_unique_or_drop(this: Self) -> Option>> { > // We will manually manage the refcount in this method, so we disable the destructor. > let this = ManuallyDrop::new(this); > // SAFETY: We own a refcount, so the pointer is still valid. > - let refcount = unsafe { this.ptr.as_ref() }.refcount.get(); > + let refcount = unsafe { &this.ptr.as_ref().refcount }; > > // If the refcount reaches a non-zero value, then we have destroyed this `Arc` and will > // return without further touching the `Arc`. If the refcount reaches zero, then there are > // no other arcs, and we can create a `UniqueArc`. > - // > - // SAFETY: We own a refcount, so the pointer is not dangling. > - let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) }; > - if is_zero { > - // SAFETY: We have exclusive access to the arc, so we can perform unsynchronized > - // accesses to the refcount. > - unsafe { core::ptr::write(refcount, bindings::REFCOUNT_INIT(1)) }; > + if refcount.dec_and_test() { > + refcount.set(1); > > // INVARIANT: We own the only refcount to this arc, so we may create a `UniqueArc`. We > // must pin the `UniqueArc` because the values was previously in an `Arc`, and they pin > @@ -456,14 +450,10 @@ fn borrow(&self) -> &T { > > impl Clone for Arc { > fn clone(&self) -> Self { > - // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is > - // safe to dereference it. > - let refcount = unsafe { self.ptr.as_ref() }.refcount.get(); > - > - // INVARIANT: C `refcount_inc` saturates the refcount, so it cannot overflow to zero. > + // INVARIANT: `Refcount` saturates the refcount, so it cannot overflow to zero. > // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is > // safe to increment the refcount. > - unsafe { bindings::refcount_inc(refcount) }; > + unsafe { self.ptr.as_ref() }.refcount.inc(); > > // SAFETY: We just incremented the refcount. This increment is now owned by the new `Arc`. > unsafe { Self::from_inner(self.ptr) } > @@ -472,16 +462,10 @@ fn clone(&self) -> Self { > > impl Drop for Arc { > fn drop(&mut self) { > - // SAFETY: By the type invariant, there is necessarily a reference to the object. We cannot > - // touch `refcount` after it's decremented to a non-zero value because another thread/CPU > - // may concurrently decrement it to zero and free it. It is ok to have a raw pointer to > - // freed/invalid memory as long as it is never dereferenced. > - let refcount = unsafe { self.ptr.as_ref() }.refcount.get(); > - > // INVARIANT: If the refcount reaches zero, there are no other instances of `Arc`, and > // this instance is being dropped, so the broken invariant is not observable. > - // SAFETY: Also by the type invariant, we are allowed to decrement the refcount. > - let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) }; > + // SAFETY: By the type invariant, there is necessarily a reference to the object. > + let is_zero = unsafe { self.ptr.as_ref() }.refcount.dec_and_test(); > if is_zero { > // The count reached zero, we must free the memory. > // > @@ -775,8 +759,7 @@ pub fn new_uninit(flags: Flags) -> Result>, AllocError> > // INVARIANT: The refcount is initialised to a non-zero value. > let inner = KBox::try_init::( > try_init!(ArcInner { > - // SAFETY: There are no safety requirements for this FFI call. > - refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }), > + refcount: Refcount::new(1), > data <- pin_init::uninit::(), > }? AllocError), > flags, > -- > 2.51.0 > > Reviewed-by: Elle Rhumsaa