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 DA03F33938C; Tue, 1 Sep 2026 13:59:14 +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=1788271156; cv=none; b=K6MYaFHtc0Zoa3LyGVMNMnLrTtuffM9+TVnmSNeGniyjwfkTaZUzLaq3fED39TBvUFHf/UxN6BSkfeR3P2R070ElhfBZERgqOLL7MCE+0E1fcN7Fvu2bxKSoZX7rzzY/iG9ewoa0e/gn6mGS1tQRoq3SNIo8uV/vlAOWy2wb180= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788271156; c=relaxed/simple; bh=L2aleJxRKrFW1w7IsLbcpUAM9hzfRgXDejME8sOErVY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=uREWL1vq5bGVleqoKhas9v0fS91K9rBOszWfQCS/Bk7zcukKBxeEAd6/IIS4Vf22S4AECLR2yqUWEGjdhGHratnSiEnGB7S1SDRiLoNKAI06u2HwkvF9vYV3/zxfbm+qEQpMP/eUgVvbby1S8SVWATm7L9EOKStwdL/WC80N1WM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Ug+tFPfl; 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="Ug+tFPfl" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 41B001F000E9; Tue, 1 Sep 2026 13:59:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788271154; bh=e4NZGL9sW0yTZysi364rSyA0YFBhdTM0YG9uCeDwPSE=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Ug+tFPfl+VDaefaVU/4BHgnPy4Ce554caP+BlVg3VmC2CafOtgwHuapsFUIbT00AM JhBz78XPBoE1K7hRx8/zC063NUrEq5FGMqx6wpPftXQthG2CT9G0g3URZMrE4U0TIY e3zCkZDksY2EmW26k9Gyy6Z5pr54TGS3qGDEduTJMHYJF/pL4zzJX8h82ZWL7H2SYd zj0S2+Rm1ob2cjq2uV3T3jXfn+GY2AORNNkiA/y9hy2O4Bu7U0lfSAYSM8FapRpaHM BKqjJ4dChKTHB63GV2bJjWfzK31GlWjYVs3pb8wPL2DheszV97Ty3+skGtidZzaHYM J5jlsijPojrWw== Message-ID: <47aa459f-27d5-4b2f-9eb9-36ebca63d890@kernel.org> Date: Tue, 1 Sep 2026 15:59:09 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/slab: disallow kfree_rcu_sheaf() on PREEMPT_RT again Content-Language: en-US To: Sebastian Andrzej Siewior Cc: Harry Yoo , Clark Williams , Steven Rostedt , Andrew Morton , Peter Zijlstra , Alexei Starovoitov , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, syzbot+acf142088e0182172e58@syzkaller.appspotmail.com, ThangNN99 References: <20260831-b4-kfree_rcu_hotfix-v1-1-4f0fb882638b@kernel.org> <20260901073339.uyKHXWCX@linutronix.de> From: "Vlastimil Babka (SUSE)" 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: <20260901073339.uyKHXWCX@linutronix.de> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 9/1/26 09:33, Sebastian Andrzej Siewior wrote: > On 2026-08-31 18:02:38 [+0200], Vlastimil Babka (SUSE) wrote: >> This partially reverts commit 2a8bb29ec9b2 ("mm/slab: allow >> kfree_rcu_sheaf() on PREEMPT_RT"). It was based on an assumption that >> local_trylock() is safe on PREEMPT_RT from any context. > … >> Signed-off-by: Vlastimil Babka (SUSE) > Reviewed-by: Sebastian Andrzej Siewior > >> --- >> Incidentally I have posted a RFC [1] that leads to replacing that >> kfree_rcu() from set_cpus_allowed_force() but now after back from >> vacation I need to check the feedback and based on this bug report I can >> already see it makes the same bad assumption that trylock is fine. > > free_to_pcs() has still this trylock. > > What I am not so sure how good is that kfree_rcu_nolock() may allocate > memory for the sheaf if there is none around. Per sashiko review it's actually bad too under the pi_lock, because GFP_NOWAIT means __GFP_KSWAPD_RECLAIM which can mean wakeup_kswapd() and thus also need scheduler locks. And it's not a PREEMPT_RT-only issue... > It could have a pool of X > and if it runs out, it runs out and waits until the clean up process > feeds the used sheafs back. There is fallback and the run out is not the > usual case. I'd rather not invent new pools, since there's fallback and the sheaf+barn is already a pool. Could be enough to make sure the allocation attempt is safe, i.e. use only __GFP_NOWARN. > I do remember RCU tried the same thing but then it got to the case where > HEAD had to be supplied or it had to be preemptible so could wait for > grace period and free it. I just don't remember if it had a pool pages > to fill pointers to or allocated pages if it run out. And I am too lazy > to look atm. Yeah. > Sebastian