From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-ed1-f74.google.com (mail-ed1-f74.google.com [209.85.208.74]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B92692F12C8 for ; Tue, 28 Oct 2025 21:48:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.208.74 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761688088; cv=none; b=HySNp7mxGnAsxl86FHZZ+JBOtfZA6apUm4mpaIvBMPczWwsO9u74kgL17v97cYk10KPxBOziSFjF7hUCffORtbjgcWb259R9R/U0HT046WWZUrvlOcwVrooSVIdXbLvvyQVG+rlYvbAgXoKPmL5smD8+05FRpln++CFSVo/oKLc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761688088; c=relaxed/simple; bh=xnWAo3zUl6Gff87g0btQedPx1DAW9IRA0yDqbJp/pPI=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=Dq+vL/NjqMbDNgUAigNQQUhy6L3H7KGyyaKhkCilK8KULuH56Y+TeNfc63vAjv+EJ6wYo2Y45hayt3QpqqsTIeAKOZOgxeav7qQkt6Ayqzihv8nzmO9MX4nHTaPxMJOIiGltIpAf/zg06A4nnMI5IcErpYoLTMyfvFe181NyzE4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--aliceryhl.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=NblzW3VI; arc=none smtp.client-ip=209.85.208.74 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--aliceryhl.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="NblzW3VI" Received: by mail-ed1-f74.google.com with SMTP id 4fb4d7f45d1cf-63c4999fa3dso3732943a12.3 for ; Tue, 28 Oct 2025 14:48:06 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1761688085; x=1762292885; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=a1rwNT7AoFhufoGiv/tnU9FtjWDK0AyzLDsdNZZgJak=; b=NblzW3VICih2GMr31DW3LWLKiKM2HAvSEzgep48yShOQUnwIkhVNZ89fs7QGXJL5Ch iQlz/C0z0XZOMvSRCi/dRYEkKj2BNsZ9U4OqCzkI52d+c8L4FpXRcmLrz5cOeQsAt63n q1A1Gl+R/Q4SC3RRczwePClycIpO2e+VojYguiRDAjFWvnjUdnLeJQzOoGPx2YXtlFo1 cpfCL8ceJEsQlAVGdTs250UkD7vApSQsPFp5XVbhNwk16uDnHzTH/NDmuxHPhNGxaXJu pQOJww0yUOuD/V8Ak09p5ftsB1zi1yWQjngGR+n2XULNt8UkiNTIn0BtuQdR8OYBZSt2 i/2w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1761688085; x=1762292885; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=a1rwNT7AoFhufoGiv/tnU9FtjWDK0AyzLDsdNZZgJak=; b=lciBzZ/l6/HEt/MGrDeKCz9ZDw7qtHzpfslgZEBy+D40vTrTKTKGw1+iOS18kJ0Pku 47kc8lOfyEW54uXb2C3d5QUrlWOaqKf8kPl7x5iFX6qVO4HoKe2vBgq8ikEpoFvIkbM7 VWQxtQ4M787Z2JPtrldP2jqDqDCGcmJLDBNWaW/G1Mnx8GRW3J6h8P5Sqk8SSP7jEcTK OZwBAsoLJMoUguvgQoP2gTGBMaAGPY+7+lgfK/sg5WD36M/YIAAdloDhCBN3bogu/rHT 2ZdXoXTBwW3zwVYGwG3sla3/sYjcJ5CN+/fhjjT4rtFENYmVIy5QRyYGp4z82FjMzhFX X/Jw== X-Forwarded-Encrypted: i=1; AJvYcCVYQK7URvBGim20iWStVwMhVEsr0Q2s0OtocjP36/cZPfYJ3NOpCNpzblf/azbZUtdJEGUo5Zg3qwlFWkw=@vger.kernel.org X-Gm-Message-State: AOJu0YxmZ3MN7nU/UnF5kUMJNcn59Zuyr7qWJ1BzLdXW/g8BHW0y/+t3 qGtMDLMTbOZ9QULo+IyE+PSchygHQJL5gm3mke6109D3CggcvBRVvsjMQbQYFbjnU5gxv6vgtiL bhe70LjaVLtGW+wStyg== X-Google-Smtp-Source: AGHT+IFq7poWf2sgLrm3BVNZylI1NJluksrbA9l1DBnB4++RIDBy8ODLNpGUK+5h9JR8qw+jZ9tjisMbHT4ls6A= X-Received: from edi25.prod.google.com ([2002:a05:6402:3059:b0:639:e051:c411]) (user=aliceryhl job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6402:2686:b0:639:7c6b:9745 with SMTP id 4fb4d7f45d1cf-640441c1efemr482701a12.13.1761688085213; Tue, 28 Oct 2025 14:48:05 -0700 (PDT) Date: Tue, 28 Oct 2025 21:48:04 +0000 In-Reply-To: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20251028-binder-bitmap-v3-0-32822d4b3207@google.com> <20251028-binder-bitmap-v3-4-32822d4b3207@google.com> Message-ID: Subject: Re: [PATCH v3 4/5] rust: id_pool: do not immediately acquire new ids From: Alice Ryhl To: Yury Norov Cc: Greg Kroah-Hartman , "Arve =?utf-8?B?SGrDuG5uZXbDpWc=?=" , Todd Kjos , Martijn Coenen , Joel Fernandes , Christian Brauner , Carlos Llamas , Suren Baghdasaryan , Burak Emir , Miguel Ojeda , Boqun Feng , Gary Guo , "=?utf-8?B?QmrDtnJu?= Roy Baron" , Benno Lossin , Andreas Hindborg , Trevor Gross , Danilo Krummrich , rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" On Tue, Oct 28, 2025 at 02:42:13PM -0400, Yury Norov wrote: > On Tue, Oct 28, 2025 at 10:55:17AM +0000, Alice Ryhl wrote: > > When Rust Binder assigns a new ID, it performs various fallible > > operations before it "commits" to actually using the new ID. To support > > this pattern, change acquire_next_id() so that it does not immediately > > call set_bit(), but instead returns an object that may be used to call > > set_bit() later. > > > > The UnusedId type holds a exclusive reference to the IdPool, so it's > > guaranteed that nobody else can call find_unused_id() while the UnusedId > > object is live. > > Hi Alice, > > I'm not sure about this change, but it looks like a lock wrapping > acquire_next_id(). > > If so, we don't protect functions with locks, we protect data > structures. > > If the above is wrong, and this new UnusedId type serializes all > accesses to a bitmap (lock-like), or write-accesses (rw-lock like), > then this is still questionable. > > Bitmaps are widely adopted as a lockless data structure among the > kernel. If you modify bitmaps with set_bit() and clear_bit() only, > with some precautions you are running race-proof. The kernel lacks > for atomic bit-aquire function, but you can implement it youself. > > I actually proposed atomic acquire API, but it was rejected: > > https://lore.kernel.org/all/20240620175703.605111-2-yury.norov@gmail.com/ > > You can check the above series for a number of examples. > > Bitmaps are widely used because they allow to implement lockless data > access so cheap with just set_bit() and clear_bit(). There's nothing > wrong to allocate a bit and release it shortly in case of some error > just because it's really cheap. > > So, with all the above said, I've nothing against this UnusedId, > but if you need it to only serialize the access to an underlying > bitmap, can you explain in more details what's wrong with the existing > pattern? If you have a performance impact in mind, can you show any > numbers? > > Thanks, > Yury Hi Yury, This does not change the locking requirements of IdPool at all. Both before and after this change, acquiring a bit from the pool uses the signature &mut self, which means that the caller of the method is required to enforce exclusive access to the entire IdPool (doesn't have to be a lock - the caller may use any exclusion mechanism of its choosing). In the case of Rust Binder, exclusive access is enforced using a spinlock. In the case of the examples in IdPool docs, exclusive access is enforced by having the IdPool be stored in a local variable that has not been shared with anyone. It's true that the underlying bitmap supports lockless/atomic operations by using the methods set_bit_atomic() and similar. Those methods are &self rather than &mut self because they do not require exclusive access to the entire Bitmap. But IdPool can't provide &self methods. The existing acquire_next_id() method has a race condition if you tried to perform two calls in parallel. But even if we changed it to perform a correct atomic bit-acquire, the fact that IdPool is resizable also incurs a locking requirement. The only purpose of this UnusedId change is to make use of RAII to automatically clean up the acquired ID in error paths. I avoided setting the bit right away for simplicity, but setting the bit and unsetting it in error paths via RAII would also work. But there would still be a lock in Rust Binder that protects the bitmap without this change. Alice