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 97FC7CD98EE for ; Wed, 17 Jun 2026 05:38:29 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 6E20E6B0099; Wed, 17 Jun 2026 01:38:28 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 66CA76B009D; Wed, 17 Jun 2026 01:38:28 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 533556B00A0; Wed, 17 Jun 2026 01:38:28 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id 16BE06B0099 for ; Wed, 17 Jun 2026 01:38:28 -0400 (EDT) Received: from smtpin20.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay10.hostedemail.com (Postfix) with ESMTP id 7028DC08E9 for ; Wed, 17 Jun 2026 05:38:27 +0000 (UTC) X-FDA: 84888299454.20.851FD24 Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf28.hostedemail.com (Postfix) with ESMTP id AF4D2C0002 for ; Wed, 17 Jun 2026 05:38:25 +0000 (UTC) Authentication-Results: imf28.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b="kRlZI/S/"; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf28.hostedemail.com: domain of vbabka@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=vbabka@kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1781674705; 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:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=BziNteM5YVB6ogs4Q9Gd0LtktaFtRuTFTklo5cSHf2o=; b=EnUFnfuhb0r3HhdphinVLTB+G8aP1BXf6RzQmSJElLaRIgY9+lqYGPmBIRoTOGKbGzwBGz e2hqy5pZW89Y+iZwuUM1HXblf2xErpORQ8IJgiZ40cinuuhrNHit300EXGlaOB3M3VloTB 0PPNOf3W+EqC3oLhwrz9LDYbTSArlKU= ARC-Authentication-Results: i=1; imf28.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b="kRlZI/S/"; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf28.hostedemail.com: domain of vbabka@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=vbabka@kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1781674705; b=aX2Qy2EgTiKi2S3ezJm0vUIqUVlRpt49p+t8Ow+s0aVvKF/JN2LJe9LYD1zDI97BN0KFeR 2v8kmo3x3/rrZ2ItyR0/Es90YCbbESCsBh2lJxVHmp1K4eAfwJal6kINtRPb1t9zmQ5CFj d6HZeSrokDrGBnT9iGyOUc8p9FWXBrY= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id 3963D600B0; Wed, 17 Jun 2026 05:38:25 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 056D31F00A3A; Wed, 17 Jun 2026 05:38:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781674704; bh=BziNteM5YVB6ogs4Q9Gd0LtktaFtRuTFTklo5cSHf2o=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=kRlZI/S/SFJ7eCHH30KOn3xKjAraKHzfGJ0VFaXesYzWx9f8/ZZX9aOtIC5gDNtsV EzVzvviG/lHxv5MJLbJt1K3PRAtgBkG746r+esGVf+fmcWatIYPqja3wSwGOyRSrxz EVx7KpbWmC5lCfbOFcd2nqfUxm2RvtQQf7p5B5UG5eL6+jFeCdW4DvUQxPp6JK6bSB 89m2xNrHdH6k/eM9OBvI8GimRTvGRKkyArQHOinpuvLSiLCOMEYllllqcxJrs3fzW1 d2MvE+tcdh10NAQc1gKhSlxxp4QK7ZU0tPLjbkDcDRxR0iTiVuEVi9nT/wvr3MM62e Nr4//4MN5llxA== Message-ID: <9d8ca1ed-bc53-417f-944b-d78eecd1e6a5@kernel.org> Date: Wed, 17 Jun 2026 07:38:16 +0200 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: Harry Yoo , 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 , Peter Zijlstra 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> <40591888-3a87-433e-b3d2-cda1cab543be@kernel.org> From: "Vlastimil Babka (SUSE)" Content-Language: en-US Autocrypt: addr=vbabka@kernel.org; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSNWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBrZXJuZWwub3JnPsLBsAQTAQoAWhYhBKlA1DSZLC6OmRA9UCJPp+fM gqZkBQJqFFy6GxSAAAAAAAQADm1hbnUyLDIuNSsxLjEyLDIsMgIbAwUJGtCBUAULCQgHAwUV CgkICwUWAgMBAAIeBQIXgAAKCRAiT6fnzIKmZJIUEADFx/tREzUImHrEwVHeSvDFmA7tJysI UVrlvrM09E7GIuzphzv7jYmo8n3ANpCczLEVr4G0syYQdTigaZgv3+FQDIIzhKih1IHhu1Ei XHlywNWKnQxxQEUNi5Mwx43wQz5XVw9F1A7gtKBKNtfogO511hAbrzagrYajyQacEJ/+sfhZ 9Da8ltHIXD8pcYaHUfQgEusCgmEd9+KrUwrTbckFKmYq5chuE6yJ4J0EmWknL096jIE6CnzF FRslQ3B1UKDjxVsm1ZHfir5NeWszLkTvGFsddFaWTgh8UycESG6VQzKXjjewXu2pG7YQYRpj QKm1W5X2TkwWkXRBZTmfmbhxIUMh3+zf5wQ463rSmDN/8v81tdqBtAW6rH/kzg1GvkaTHXn0 507yEHFzBksk2viAuIxxr7km8+/KARYLIdGtx30EG8cKzAUZOK6WqxtNCsXUJNrVE8CWrCaD icoNu7Fs1c5hmPHdSTnU48ce67449DdnO4neLSNhRiGlMHJgfJUmgrxu/hcYeOZ3haWmEQ2w uW1Mh01OHi8QZHCEyAbABrPs9GUgccc/4eYXX9hIgxfSkYzn8f+8NuIFPWl/0uTvjgqU29FQ SbzOLxHq9439Ox40G5mS5eZXRGxITYR+6TXvRGI6P/264jvflnr/pDGUttaikU+0W+1uxgKH cmYbEc7ATQRbGTU1AQgAn0H6UrFiWcovkh6EXVcl+SeqyO6JHOPm+e9Wu0Vw+VIUvXZVUVVQ La1PQDUi6j00ChlcR66g9/V0sPIcSutacPKfdKYOBvzd4rlhL8rfrdEsQw5ApZxrA8kYZVMh FmBRKAa6wos25moTlMKpCWzTH84+WO5+ziCTsTUZASAToz3RdunTD+vQcHj0GqNTPAHK63sf bAB2I0BslZkXkY1RLb/YhuA6E7JyEd2pilZOrIuBGl/5q2qSakgnAVFWFBR/DO27JuAksYnq +aH8vI0xGvwn75KqSk4UzAkDzWSmO4ZHuahKtQgZNsMYV+PGayRBX9b9zbldzopoLBdqHc4n jQARAQABwsF8BBgBCgAmAhsMFiEEqUDUNJksLo6ZED1QIk+n58yCpmQFAmfIHFQFCRYU6J8A CgkQIk+n58yCpmS2PA//bqN1LfcotmArgElsa+0EGZSQlYgK48pm8WAeTXTngudP9IJ4SuKY HR5RNjHcBeqN+Me0zxRqYzRb8nGanHEkDyf4Im8DQM8d6vbyU+FcPmG4skud4kgS1zMHnlVd SXfSIwKC/hKgdHG8aBV7545Lz9X6Iohea+94wneD0aw/hqF+QWewGZhWJriWAZtvEkzNjQOi 4U9F/trLten/x7bpphDSnDMKJtITbtzATT1Dq7o7VpIUK1nCTQALMuMjKCdi8OdU/+V+R3O4 0PXWvX8qrvqYapVbZ+9KqT74FsuB0Ya9uXwgBF2Q6cRuETZk5vqaqKxzqoQZCO8AOz/58j6O 2RHNy/mZEN+7tJ5Tsq42zVJ4jxsT8b9YplavCMsnBgDeRWhcbYhCyttoL7nYISyWg4kQYZ/P wIV3OuNv2f8iKYsxNsRuClOAF82+gvqOy1/1pprFjy8uo2pkoOrb63aOP3vO5VHnRKgra6dq NcaZ+c6J4H+nEJGi2SkHAUJz5oBzuThvPudLvPA/SK8sKoM01IRxSihev/S/5WLazXB1PGem OCbvzC1IjWJJraxiDJ5IygokapUa2RP7+WBR22skQ3SSl6G107QgWKSyTOGWEaRmV53vxQLV jXuCmzSSasTL60zq5yGrT4/DYQVSNEUiUbG4pYekxJujNeEDkUlky0Y= In-Reply-To: <40591888-3a87-433e-b3d2-cda1cab543be@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: AF4D2C0002 X-Stat-Signature: ys5bksaxnhjhfsbebo8d8gkn73nf9isw X-Rspam-User: X-HE-Tag: 1781674705-60859 X-HE-Meta: U2FsdGVkX1+q+zB6kQQM8LGafKUaILvG+DyRbbuUMOsvscHkfnsBVy/JPj/ZU4LjlwwgJbThjepAYDfcRtIMYyt5Hx2KRhJdKmI4/TEOLmOYw67w5PXiSi2GjgqadAECzrzntr6imXg/aWaDQk1irlCeSuGfH/6hBJorPVSuf9bZyyYvw/ONynGM9Y1Wcu4lgXns7NofNBcIxmVvVU/6RLIDpi5chhr+BdHu3wO3mJWaN7Zjft6755CD1E+7vtymwHwioR/k4d76AlXskEPAOJQVzB9SVC8WM2Pd+s0Qe56zCvz4RRlZR9PimndI/CTwzTygnL4tFBwqp8B+ZLbclRLATTQ0PRQ8yeHcnF8NLhq342Jy6cqawDvlxZmbOW3i0IMu/K832Nhw7sbm++om+GeUj8dzAXm/QysWKn+1U9PGEYd7N/SkeRqRUcU4IT/JuUcHOEyg9dem/okMBS8rhgoD9Me5sy8H6KSb5StRoXGTV4SzEuHs1oUPU76kYsioNYu7Ds0bPthWWunAuci0nKANNPO3xbTeYi6iBgRY9VtguKWSZoaJZxMCgWwrtd+2/KPyw01wMy0PPwwQGMLFlKGBeL4gh6vOwBhcIQE6lju5a8qjcixtTSGI+E9mpBgHUUOb3scM5yRAVkJBcdHlTTZad+oWnKd32iFFg91CF7dftmNNG1AFYhKz0kzJV42K+T6spowGnOIPH4vanuMdnwcyAckRqd4Wbh+FF5XgtiJEGi0GyMo2e/tSnGn183iKoGxKsGerPR1vXz+7BMdhNXm2X4YXJylLRJaJmveeDhXq38XWS9QTzlkYsLfScTumF+tTdUMqckDk3dWUt/jsBEbzsVoaUW5Pvywvpm6JTuM+4itWk19RGX7lsW3/cCGtJHXhynwddLn954eu7AXcwkNLJffo0Nz4I1Zy2GeKyXMRPmEdpU/Y070YaEJi+80KF08v9/7l1sFTQjdS89F PQ4t4QBo P/QvaQYrHLVPBHaYykWLAl7Bvs5evY8CXmUMCy3w65HY4tG4780oEIy7z8nC55gLi+8dtosB+VBRy6O1WFMZ1UBiALiYDGY7B2NipfqxaJfwa2UXqkeIa9XIAWjZQ8gP24ATBxXATXvXyrr4dGhF2Q1svIhOA5xxaxOXHYwgv7/fyc1sFkqoVGVjB+zVN2W28V+lHDHo3ou5ssgnuhMWHyiFRI6tNm4Q5N5zq6Mv6/+X4rPxHw1DVhTsdjPsZXb5xmvLgcXSF4l2nSMXon5vroq1RS3Btor6IqdKHYAQ/TwWRUif7IB+4IHXUqDaBmWUYRsyugi1ZSiLw6Pk= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/17/26 07:14, Harry Yoo wrote: > > > 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_RT. >>> 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 = 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 lockdep >>> map on PREEMPT_RT to avoid suppressing valid lockdep warnings. >>> >>> Link: https://lore.kernel.org/linux-mm/6811cc17-8ee4-48c8-8cbf-6bf4d9f98162@kernel.org >>> Suggested-by: Vlastimil Babka (SUSE) >>> Signed-off-by: Harry Yoo (Oracle) >> >> 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 to >> 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()? Sounds good. > 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(): Looks like the case. Well if the fallback path of kfree_nolock() that is irq_work_queue() is indeed safe here. >> /* >> * Because this is called with p->pi_lock held, it is not possible >> * to use kfree() here (when PREEMPT_RT=y), therefore punt to using >> * kfree_rcu(). >> */ >> kfree_rcu((union cpumask_rcuhead *)ac.user_mask, rcu); > > Any thoughts, RCU/RT folks? >