From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 6CC87399357; Wed, 17 Jun 2026 05:38:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781674711; cv=none; b=Z13enEo8s4OGtX4Z2+j8cjw09OMeoxrNZSfkggcNIMsHk5a2Ach/iCyt/bnRtMv2oO2wrcuUShPF0yxOthIcLQGc8c6NB3gAe83jHLIBQYp+BTUOt2T0xqI7/Au7qnzu+4YXT0Eqx1Oex60MJE+X8NHNp/fZIdh/OLL0401ya70= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781674711; c=relaxed/simple; bh=R9EJveHGxrLDMK3qG6qDZwx/iAI+usRoD/JIjaVnaG0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=N3fKuiNaLMLBRRztAcy+r5CiAZDRgF4xeH7aar33FrFAh68t3pZwEQEDbn2z83epq7FYL0wgEMwDdSZUDpwUDFcx2E+R5soqmCEjZbTPbqOR/qHbTBU8UJqwf7dSupyi6zItA3p84SM2bupf03vZUL0aGbITSKMR2x0Qm5Ty1Gw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=kRlZI/S/; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="kRlZI/S/" 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 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 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? >