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 0859A1F4CAB; Thu, 12 Jun 2025 08:47:56 +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=1749718077; cv=none; b=aU7modrsOIBnRmDR6AqIftZyF3MgzU4LUq9Jrd5XaYQ2ZLg4LHaqZjQqcUbqTKXF40s2/+s94nBdKbHm6YMXIq2FrS7fUAfblT/WG5occyDUeBrgTBDNkYjUth4spEGi65eksNsDRQTZKmisQNmR97VoQleR5lESxBKuUSnBPAE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749718077; c=relaxed/simple; bh=GueOvweik3rNbhheZCa910uQPXb79aqCUMGeUSjUtHs=; h=Mime-Version:Content-Type:Date:Message-Id:Cc:Subject:From:To: References:In-Reply-To; b=FtiJ0ELeaQxYW13ySZNtU/x+6IYGD6qza/YVxh3fw/sEIKXCUNVyeBTlAMrYqYRkCCnqB/vjwFXzXeU2YXGdFmibXrbpIieH5UMG5RrvEJwHLy1sxFPzDa+xjg7XadwF6S5tSg/gDyF7D67ZdNJMmGMSMoojzBI9PeKmAJxyX7A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=X8UmPG2x; 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="X8UmPG2x" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B94A7C4CEEA; Thu, 12 Jun 2025 08:47:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1749718076; bh=GueOvweik3rNbhheZCa910uQPXb79aqCUMGeUSjUtHs=; h=Date:Cc:Subject:From:To:References:In-Reply-To:From; b=X8UmPG2xp5OL9kkQ5BBbWTfTyjSXDNGTAHEBALa0lmjbls9NmFXbXH82YRc5AFfUg ctoK7FCb9O9YkXx+fshddAfWTV3DS6GB06jyBjTYakd7NqwP5/0l11Wi//lkIxf2OK GZVlWgmc/piJJFKuHi8HNFDvaCfohNmRFyKl0+jLj+S3b/iyoGJXoWgAIYxvjRswgW cgslof9HXZC6Dk83wVym4JNigrL5Z0YKzNDrjeFSpP/f35763IVaCBSBDg/r6Xyk7E t7lNE6tg5Oi+MWGEHwzgVIpFtPBETa/eV5K3fRDkwYg1qVoZ9kwFybHbEq4IFJQhTT mgC2Rp/+l4N5g== Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Thu, 12 Jun 2025 10:47:51 +0200 Message-Id: Cc: "Danilo Krummrich" , , , , , , , , , , , , , Subject: Re: [PATCH 3/3] rust: devres: fix race in Devres::drop() From: "Benno Lossin" To: "Alice Ryhl" X-Mailer: aerc 0.20.1 References: <20250603205416.49281-1-dakr@kernel.org> <20250603205416.49281-4-dakr@kernel.org> In-Reply-To: On Thu Jun 12, 2025 at 10:15 AM CEST, Alice Ryhl wrote: > On Thu, Jun 12, 2025 at 10:13=E2=80=AFAM Benno Lossin = wrote: >> On Tue Jun 3, 2025 at 10:48 PM CEST, Danilo Krummrich wrote: >> > In Devres::drop() we first remove the devres action and then drop the >> > wrapped device resource. >> > >> > The design goal is to give the owner of a Devres object control over w= hen >> > the device resource is dropped, but limit the overall scope to the >> > corresponding device being bound to a driver. >> > >> > However, there's a race that was introduced with commit 8ff656643d30 >> > ("rust: devres: remove action in `Devres::drop`"), but also has been >> > (partially) present from the initial version on. >> > >> > In Devres::drop(), the devres action is removed successfully and >> > subsequently the destructor of the wrapped device resource runs. >> > However, there is no guarantee that the destructor of the wrapped devi= ce >> > resource completes before the driver core is done unbinding the >> > corresponding device. >> > >> > If in Devres::drop(), the devres action can't be removed, it means tha= t >> > the devres callback has been executed already, or is still running >> > concurrently. In case of the latter, either Devres::drop() wins revoki= ng >> > the Revocable or the devres callback wins revoking the Revocable. If >> > Devres::drop() wins, we (again) have no guarantee that the destructor = of >> > the wrapped device resource completes before the driver core is done >> > unbinding the corresponding device. >> >> I don't understand the exact sequence of events here. Here is what I got >> from your explanation: >> >> * the driver created a `Devres` associated to their device. >> * their physical device gets disconnected and thus the driver core >> starts unbinding the device. >> * simultaneously, the driver drops the `Devres` (eg because the >> driver initiated the physical removal) >> * now `devres_callback` is being called from both `Devres::Drop` (which >> calls `Devres::remove_action`) and from the driver core. >> * they both call `inner.data.revoke()`, but only one wins, in our >> example `Devres::drop`. >> * but now the driver core has finished running `devres_callback` and >> finalizes unbinding the device, even though the `Devres` still exists >> though is almost done being dropped. >> >> I don't see a race here. Also the `dev: ARef` should keep the >> device alive until the `Devres` is dropped, no? > > The race is that Devres is used when the contents *must* be dropped > before the device is unbound. This example violates that by having > device unbind finish before the contents are dropped. If `Devres::drop` is being run, nobody has access to it any longer. Additionally, the data in the revocable has already been dropped if `revoke()` has been run, so it's fine? --- Cheers, Benno