From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 1FE93CD98E1 for ; Wed, 17 Jun 2026 05:14:45 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id CCA6C6B0096; Wed, 17 Jun 2026 01:14:44 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id C7BDA6B0099; Wed, 17 Jun 2026 01:14:44 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id B6ABB6B009B; Wed, 17 Jun 2026 01:14:44 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id 769F46B0096 for ; Wed, 17 Jun 2026 01:14:44 -0400 (EDT) Received: from smtpin29.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay05.hostedemail.com (Postfix) with ESMTP id E3F7F4033D for ; Wed, 17 Jun 2026 05:14:43 +0000 (UTC) X-FDA: 84888239646.29.FA57334 Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf12.hostedemail.com (Postfix) with ESMTP id 4192340004 for ; Wed, 17 Jun 2026 05:14:42 +0000 (UTC) Authentication-Results: imf12.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=Vq5KoxGj; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf12.hostedemail.com: domain of harry@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=harry@kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1781673282; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=i4gI8nBuxA9HRcUwNvotK2yH1/Y4niKmJ4bOseHnoOY=; b=cRPzY6iLC4oFUbLinKqEHooTATAjYUC9imOjijFaypjWbXdvkKu52RbO+wM6aAgVyUeiQu xtQLoJb1I9WmU6LP1pSxTUnm6Io2q2HW/27ysR5K8CCGu1pnn0rZkv15JPo9Tqlvis0GmD gHkuzX7RltgoJtVv+U1vjghdKkG8oNw= ARC-Authentication-Results: i=1; imf12.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=Vq5KoxGj; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf12.hostedemail.com: domain of harry@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=harry@kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1781673282; b=GzeQHyNOfUuEfRilROuElyG45uSqFc7gGe4wq4Pvmk64jT4rEILlKDAfmRAwLyagn1xBZg h7/NFe8NlHmgUk31W7OTXVDOYjVlWuitn+ASaqXSU554wn7HC9niBFuBMgHN76lnoeq9iz 2FHBI/hdXo/gKOkvwDrxjKTSt4pMRXU= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id 75B24600AA; Wed, 17 Jun 2026 05:14:41 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5991F1F000E9; Wed, 17 Jun 2026 05:14:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781673281; bh=i4gI8nBuxA9HRcUwNvotK2yH1/Y4niKmJ4bOseHnoOY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Vq5KoxGjOeZU7bfYcs6X2ic2+hFhWSKqGgqfdCjVZy2jo1ZH3yXx6tBFJUGVnDLPq FRkbuYQdKulgt+hMMWNOJ6rg/hgsSsWtQgRZBsc8wIALCQdjtTY+90vF3lTjx+CJ76 gc7RwVd0S2MmHruzJzyn/jJf3x+enPN8Yk/c3tVX11PsHcEakhw30+CDNpaiGZxbLi vo2iyxTE4uBv23I3XtYp5Km71YP+rjdtnogZPG5Ynv8IuBnmu/0SrQfqKiazfIrc2K MzKx0UlxQIeCPhRWYE7Rlj1opOvG/qB7x+PPd2g3llsJgPCPP5lcop1iW+bs50TTtT 3GVL74u3KPyoA== Message-ID: <40591888-3a87-433e-b3d2-cda1cab543be@kernel.org> Date: Wed, 17 Jun 2026 14:14:16 +0900 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH for-next v3 6/9] mm/slab: allow kfree_rcu_sheaf() on PREEMPT_RT To: "Vlastimil Babka (SUSE)" , Andrew Morton , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , Alexei Starovoitov , Andrii Nakryiko , Puranjay Mohan , Amery Hung , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Pedro Falcato , Suren Baghdasaryan Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, rcu@vger.kernel.org, bpf@vger.kernel.org References: <20260615-kfree_rcu_nolock-v3-0-70a54f3775bb@kernel.org> <20260615-kfree_rcu_nolock-v3-6-70a54f3775bb@kernel.org> Content-Language: en-US From: Harry Yoo In-Reply-To: Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="------------kEBv9bKx3bhSjfXvp2FnnpK0" X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 4192340004 X-Stat-Signature: ea8uqdqxu48be4obfed85a1i74ppeh6r X-Rspam-User: X-HE-Tag: 1781673282-643878 X-HE-Meta: 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 j3zA94wq 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --------------kEBv9bKx3bhSjfXvp2FnnpK0 Content-Type: multipart/mixed; boundary="------------vb0IaWkZAOplq9hp2tUYiTMN"; protected-headers="v1" From: Harry Yoo To: "Vlastimil Babka (SUSE)" , Andrew Morton , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , Alexei Starovoitov , Andrii Nakryiko , Puranjay Mohan , Amery Hung , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Pedro Falcato , Suren Baghdasaryan Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, rcu@vger.kernel.org, bpf@vger.kernel.org Message-ID: <40591888-3a87-433e-b3d2-cda1cab543be@kernel.org> Subject: Re: [PATCH for-next v3 6/9] mm/slab: allow kfree_rcu_sheaf() on PREEMPT_RT References: <20260615-kfree_rcu_nolock-v3-0-70a54f3775bb@kernel.org> <20260615-kfree_rcu_nolock-v3-6-70a54f3775bb@kernel.org> In-Reply-To: --------------vb0IaWkZAOplq9hp2tUYiTMN Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On 6/17/26 2:24 AM, Vlastimil Babka (SUSE) wrote: > On 6/15/26 13:06, Harry Yoo (Oracle) wrote: >> As suggested by Vlastimil Babka, kfree_rcu_sheaf() can be used >> on PREEMPT_RT if we always assume spinning is not allowed on PREEMPT_R= T. >> This is because local_trylock and spinlock_t are safe to use with >> trylock variant as long as the kernel does not spin and the context is= >> not NMI and not hardirq. >> >> Now that __kfree_rcu_sheaf() knows how to handle allow_spin =3D false,= >> relax the limitation and try the sheaves path on PREEMPT_RT as well. >> >> Keep the lockdep map on non RT kernels. However, do not use the lockde= p >> map on PREEMPT_RT to avoid suppressing valid lockdep warnings. >> >> Link: https://lore.kernel.org/linux-mm/6811cc17-8ee4-48c8-8cbf-6bf4d9f= 98162@kernel.org >> Suggested-by: Vlastimil Babka (SUSE) >> Signed-off-by: Harry Yoo (Oracle) >=20 > LGTM, but maybe unnecessary pessimistic wrt call_rcu() on PREEMPT_RT? > I thought (in the Link: above) we'd only need to downgrade allow_spin t= o > false on PREEMPT_RT for handling sheaves movement from/to barn and > alloc_empty_sheaf(), but call_rcu() would be safe from kfree_rcu() even= on > RT? Indeed. Good point, thanks! Hmm, but I'm not sure that it's worth the complexity given that PREEMPT_RT tries very hard to avoid disabling IRQs... > Or is the irqs_disabled() condition rare enough so we don't care? Given that most users don't call kfree_rcu() under raw spinlock or IRQs-disabled section on PREEMPT_RT, I think it's okay to keep it as is (it's not making things worse, at least) and wait for call_rcu_nolock()? On a side note, I don't have much idea on what needs to call kfree_rcu() under a raw spinlock, other than set_cpus_allowed_force(), which should really be using kfree_nolock() instead of kfree_rcu() once we support kmalloc() -> kfree_nolock(): > /* > * Because this is called with p->pi_lock held, it is not possible > * to use kfree() here (when PREEMPT_RT=3Dy), therefore punt to using > * kfree_rcu(). > */ > kfree_rcu((union cpumask_rcuhead *)ac.user_mask, rcu); Any thoughts, RCU/RT folks? --=20 Cheers, Harry / Hyeonggon --------------vb0IaWkZAOplq9hp2tUYiTMN-- --------------kEBv9bKx3bhSjfXvp2FnnpK0 Content-Type: application/pgp-signature; name="OpenPGP_signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="OpenPGP_signature.asc" -----BEGIN PGP SIGNATURE----- iHUEARYKAB0WIQQQ1ub6gR5ogjaKRmOGXBN6rc5S1gUCajItKAAKCRCGXBN6rc5S 1jLFAQDJjkqWNsckxRckQG/ifpkFidi3A6aFYgVvzxCG+/hQQwD+LUnZJI7/+UOa K4FDOVTUA6hwmP3OMAXM1Q1fH4PjOwM= =nbP0 -----END PGP SIGNATURE----- --------------kEBv9bKx3bhSjfXvp2FnnpK0--