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 B6F4D517BAF; Wed, 30 Sep 2026 18:22:50 +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=1790792572; cv=none; b=E/GAMkmm+33Zdu3uk1Y6OV//tKpgvKjM8euOjzjznuI2Fpr7Wp5aKpmcObEuAHBUwoijorISCsJLTR4k9H7KhHKIIrjGZyVRKca8yE5bSZsAvvDM2k6PMnO6JfdHCMOxpT17875MxEc/jMGXfE3+UqNykJNmgx+sy4fJ6Inin6I= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790792572; c=relaxed/simple; bh=Fxb2PL0LrvsWAnIgtE9NDrp8X2DvEKXST2VKwXE8NlA=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=bL+UsQ2cB98Uz2r/IfdOLpHbzC0VkmODurzkmjVryNjXkwGJiR1Uj3jVlRZwx0w+9/GOlwKDtrCu9rgrq4lWos+Bcp56raYh12ir/OVVNo93M08H6t5Mpk4dxgIYabq0r218ta8aaYQTRSfxLYDjFY/qOgP1gyJ6JsmFOIPJqa4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=bXMpd0RU; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="bXMpd0RU" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EB27D1F000FF; Wed, 30 Sep 2026 18:22:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1790792570; bh=gNwJCHPNfaLWrCPRa1HypBOAl90z5tm4kz/MPVdnV4Q=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=bXMpd0RUVegu/G+X6yXh7d15ghvERPZFlLMsV4g5GkU6qwPb9t4A7EbbkgethOcMe MVWjHsP2JfaKMpTaANRQ43oQKx6MkvDGEImh9OPY9KV/PN13EE5L87NyJd5OWPsU7C U92WCVPOXMVJ29MGZ72cTLOuUODZ4bHTuxkQ86wM= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Josef Bacik , Andrew Morton , Tejun Heo , Roman Gushchin , Jan Kara , "Lorenzo Stoakes (ARM)" , David Hildenbrand , Dennis Zhou , "Liam R. Howlett" , "Matthew Wilcox (Oracle)" , Michal Hocko , Mike Rapoport , "Paul E . McKenney" , Suren Baghdasaryan , Vlastimil Babka Subject: [PATCH 5.15 700/752] writeback: report a Tasks-RCU quiescent state per cgwb drain pass Date: Wed, 30 Sep 2026 17:29:30 +0200 Message-ID: <20260930152413.379107763@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260930152358.131179731@linuxfoundation.org> References: <20260930152358.131179731@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 5.15-stable review patch. If anyone has any objections, please let me know. ------------------ From: Josef Bacik commit 407a5d205179a4ab186571b0e16ec42725dc77bc upstream. cleanup_offline_cgwbs_workfn() drains a dying cgwb by calling cleanup_offline_cgwb() until it returns false, with a cond_resched() between passes. On a CONFIG_PREEMPTION kernel that cond_resched() does nothing: _cond_resched() is a plain "return 0", and under PREEMPT_DYNAMIC the full and lazy modes disable it. Since commit 7dadeaa6e851 ("sched: Further restrict the preemption modes") those are the only two models on the architectures with PREEMPT_LAZY support, arm64 and x86 among them, so the drain loop never reports a Tasks-RCU quiescent state. A worker draining a cgwb with millions of attached inodes runs for minutes. On a 6.18 arm64 host in lazy mode the cgwb worker drained one dying cgroup's writeback domain for over 11 minutes. A BPF program unlink (bpf_trampoline_unlink_prog -> bpf_trampoline_update -> unregister_ftrace_direct -> ftrace_shutdown -> synchronize_rcu_tasks()) waited on that grace period while holding the trampoline mutex, 42 tasks queued behind it in D state, and the hung task detector fired at 614 s and panicked the host. Any BPF or ftrace detach during a long drain inherits the drain's length. Fix this by calling cond_resched_tasks_rcu_qs() so we do not stall out anybody who calls sycnrhonize_rcu_tasks(). We put this in a do { } while loop because if we have many small cgroups cleanup_offline_cgwb() will return false and we will never call cond_resched_tasks_rcu_qs(), creating the same problem. Link: https://lore.kernel.org/20260909-cgwb-tasks-rcu-qs-v1-1-967a7754771f@toxicpanda.com Fixes: c22d70a162d3 ("writeback, cgroup: release dying cgwbs by switching attached inodes") Signed-off-by: Josef Bacik Signed-off-by: Andrew Morton Link: https://lore.kernel.org/bpf/9d444098-7c03-4163-af12-bd0a79a51443@paulmck-laptop/ Assisted-by: LLM Acked-by: Tejun Heo Reviewed-by: Roman Gushchin Reviewed-by: Jan Kara Acked-by: Lorenzo Stoakes (ARM) Cc: David Hildenbrand Cc: Dennis Zhou Cc: Liam R. Howlett Cc: Matthew Wilcox (Oracle) Cc: Michal Hocko Cc: Mike Rapoport Cc: "Paul E . McKenney" Cc: Suren Baghdasaryan Cc: Vlastimil Babka Cc: Signed-off-by: Greg Kroah-Hartman --- mm/backing-dev.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) --- a/mm/backing-dev.c +++ b/mm/backing-dev.c @@ -693,8 +693,9 @@ static void cleanup_offline_cgwbs_workfn continue; spin_unlock_irq(&cgwb_lock); - while (cleanup_offline_cgwb(wb)) - cond_resched(); + do { + cond_resched_tasks_rcu_qs(); + } while (cleanup_offline_cgwb(wb)); spin_lock_irq(&cgwb_lock); wb_put(wb);