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 7613E28A3F8; Tue, 12 May 2026 09:42:07 +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=1778578927; cv=none; b=Q5CMJvyFXTJl/X/COx65FdJgtegMNCbO3Pq26jrXYj8j3+mIUCa3lEwG2xHQA2QgZPjn0KEFdJd2Y1htgcRWixbu6/+S4NonB36Ugkbvg6ORpI7ogB75ZDv41tUwQwckzWaUh19TWyl6lBQI59wi/DZIW7Zgfr/22UH381rPSk4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778578927; c=relaxed/simple; bh=TZzFpfZquyF7nqUeGNZbJf5qjfi17m7pJ6VptfMCcsg=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=EMzkyn6HjUf5yLTmNCn39tl7cvPZDLDoW9J6RsZ8lt1LmYg8+OVztVHVaHJuZVNdgr4GQRiiUZmqnN4NFpIYgYyNtIiC3/rLgwBwfioIGoD8cK5l1KmJreYLz33sfmf1Emc9e1apek/AgquZllNpCe7mcdF3RXABTRbCLP8SmUw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=pAfDWnlz; 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="pAfDWnlz" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E5A6AC2BCB0; Tue, 12 May 2026 09:42:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778578927; bh=TZzFpfZquyF7nqUeGNZbJf5qjfi17m7pJ6VptfMCcsg=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=pAfDWnlzNJx992VZLiP6Tx9+gfis9cyaQSr2iWOq5h388ES9SsYje0vyxiURr5dWp hQCOPZVHRKmkf+285wy1lfJhfYTxOCkA/k7ZBUNGa0fuiMOcMy+6BAPolXqzAL9OUL pLFDTqAcAuTAT35fOiGIY3ycA5sjMq9i7Z5fDpiV4Ca01RYe8RsVTAVCkHYzFuzWQ3 631T81u/eKY8WDkIR2qovwqNZ9s4c63vigoovOgnqbWjoZtiAXhqCinPzKelsX3ixK /zQsvVbEoCrgAcZzA3ZI3PHKz6g2p9+6yWcEEGbzpbQL4EeZuZNcEqusSwdNHZ3tQg nd4RnU/ykkoEA== From: Andreas Hindborg To: Alice Ryhl Cc: Miguel Ojeda , Boqun Feng , Gary Guo , =?utf-8?Q?Bj=C3=B6rn?= Roy Baron , Benno Lossin , Trevor Gross , Danilo Krummrich , rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] rust: sync: add lazy initialization methods to SetOnce In-Reply-To: References: <20260215-set-once-lazy-v1-1-6f5bd2efda11@kernel.org> <4EYpvxiPLRBBb2fUI9n8ZrVkve50KAvXwj7oFqoYhtpIuY_U7eVtiwnjA-ZR8n7jCwoJL67EI1ex2_DmBs5UMg==@protonmail.internalid> <875x7xdjvr.fsf@t14s.mail-host-address-is-not-set> <87zf58ddad.fsf@t14s.mail-host-address-is-not-set> <875x4t58de.fsf@t14s.mail-host-address-is-not-set> Date: Tue, 12 May 2026 11:41:58 +0200 Message-ID: <8733zx55i1.fsf@t14s.mail-host-address-is-not-set> 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 "Alice Ryhl" writes: > On Tue, May 12, 2026 at 10:39:57AM +0200, Andreas Hindborg wrote: >> Andreas Hindborg writes: >> >> > "Alice Ryhl" writes: >> > >> >> On Mon, Feb 16, 2026 at 11:26:11AM +0000, Alice Ryhl wrote: >> >>> On Mon, Feb 16, 2026 at 12:10:16PM +0100, Andreas Hindborg wrote: >> >>> > "Alice Ryhl" writes: >> >>> > >> >>> > > On Sun, Feb 15, 2026 at 09:27:17PM +0100, Andreas Hindborg wrote: >> >>> > >> Add methods to get a reference to the contained value or populate the >> >>> > >> SetOnce if empty. The new `as_ref_or_populate` method accepts a value >> >>> > >> directly, while `as_ref_or_populate_with` accepts a fallible closure, >> >>> > >> allowing for lazy initialization that may fail. Both methods spin-wait >> >>> > >> if another thread is concurrently initializing the container. >> >>> > >> >> >>> > >> Also add `populate_with` which takes a fallible closure and serves as >> >>> > >> the implementation basis for the other populate methods. >> >>> > >> >> >>> > >> Signed-off-by: Andreas Hindborg >> >>> > > >> >>> > >> + /// Get a reference to the contained object, or populate the [`SetOnce`] >> >>> > >> + /// with the value returned by `callable` and return a reference to that >> >>> > >> + /// object. >> >>> > >> + pub fn as_ref_or_populate_with(&self, callable: impl FnOnce() -> Result) -> Result<&T> { >> >>> > >> + if !self.populate_with(callable)? { >> >>> > >> + while self.init.load(Acquire) != 2 { >> >>> > >> + core::hint::spin_loop(); >> >>> > >> + } >> >>> > > >> >>> > > We should not be implementing our own spinlocks. >> >>> > >> >>> > That is a great proverb. I'd be happy to receive a suggestion on an >> >>> > alternate approach for this particular context. >> >>> >> >>> You can add a spinlock to SetOnce. Like I mentioned previously [1], >> >>> support for waiting will require the addition of extra fields. >> > >> > Thanks, I'll be sure to take a look again. >> >> I took a look at this again. I think the structure will be less >> efficient if we use a spin lock. >> >> Initialization is now >> - cmpxchg lock relaxed >> - store pointer >> - store lock release >> >> With a spin lock it will be >> - lock acquire >> - test pointer >> - store pointer >> - lock release >> >> All the other tests for initialization is now: >> - load lock acquire >> - load pointer >> >> They will become >> - lock acquire >> - load pointer >> - test pointer >> - lock release >> >> >> bit_spinlock does not make this any better. It just gives us 64 locks in >> a u64. It does not help us store state of the data structure >> (empty/populated). >> >> Do we want a less efficient data structure in order to gain benefits of >> lockdep and friends? > > I'm not just talking about lockdep. Your current implementation is wrong > in several other ways, for example: > > 1. Spinlocks must disable preemption. That is an easy fix. > 2. It doesn't fall back to a mutex under PREEMPT_RT. I don't know how to solve that, but I'm sure there is a way. > > And probably lots of other things. By using the kernel spinlock, you do > not have to worry about the long list of things spinlocks have to get > right. Nah, can't be that many things. But I get your point. >> >> By the way, back then I suggested renaming it from OnceLock to SetOnce >> >> because you did not support waiting for the value to be populated, and >> >> you said you didn't need that. If you add that feature, then we should >> >> rename it back to OnceLock, or create a new type OnceLock for users who >> >> need that additional feature. >> > >> > That is fair. This is a different use case than the original one though. >> > I think we should keep this as one type for code reuse, but I am fine >> > with renaming to something that describe the usage better. >> >> Please note that even though it could be added, we do not have a `wait` >> method now. What we have are blocking initializers. > > You may have open-coded `wait` inside of `as_ref_or_populate_with`, but > you still have the functionality. As I said, I'm fine with whatever name, but I'd appreciate if someone else chime in, so we don't have to change the name too many times. Best regards, Andreas Hindborg