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 D48F149E155; Thu, 10 Sep 2026 22:25:38 +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=1789079140; cv=none; b=rsCYeEb6u2b4lNMiRLRbJ/SEbhySy30CQ1LwF0up++cMDCMNLlEnptLIJZbC/JsG3XP89dDaWwxZNMsFfTGNaXdD/wXDaDX+5xkOuqVlY+DxjrwyXoTPF5QTNJghjMsSFv17dCHL1k+APSLaYgSVgkPhExzNcmvyFrSq2Yb03UI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789079140; c=relaxed/simple; bh=ldrpft5js/r/dFy+SlIx8PSSFYKaC2RiGYkkxVB6PPY=; h=Date:To:From:Subject:Message-Id; b=nsqFCdQgDu7BgqS2ghje6MzCEnyR0UuVxR381j2RBoxe7Rmo8w7odlNonuqym238WOYyIvq7a/Dd0g400r3ZIFXSvdgSCvYpj5zfhnRfDZvpvkfREfJcFZpprcZGMb6ZOJ9uM91qAqYz3uIr3KEHzclAbeL7zBPYqFE8PH2Ed1U= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=YiDq0VLC; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="YiDq0VLC" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 520C91F000FF; Thu, 10 Sep 2026 22:25:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux-foundation.org; s=korg; t=1789079138; bh=0+pgbX0k63l75xVSkg0dWQC20oqFPeeE/a1o3vV6TGk=; h=Date:To:From:Subject; b=YiDq0VLCH96fLeMMibe1PVQBOfQAGoF4WGap0EwHMFFZZPTkk9QlcISlLPLt5QPpZ q51yZfvKgEWY7s0LbJnezEZmOlrNwRKo70v4mDOBYrPZBmYaTc9NhK+3J0+c9CLRZ1 BcMl08XiDWUBJzy3VampwYj1YZ6DHf7ihjFm9pww= Date: Thu, 10 Sep 2026 15:25:37 -0700 To: mm-commits@vger.kernel.org,willy@infradead.org,vbabka@kernel.org,tj@kernel.org,surenb@google.com,stable@vger.kernel.org,rppt@kernel.org,roman.gushchin@linux.dev,paulmck@kernel.org,mhocko@suse.com,ljs@kernel.org,liam@infradead.org,jack@suse.cz,dennis@kernel.org,david@kernel.org,josef@toxicpanda.com,akpm@linux-foundation.org From: Andrew Morton Subject: + writeback-report-a-tasks-rcu-quiescent-state-per-cgwb-drain-pass.patch added to mm-hotfixes-unstable branch Message-Id: <20260910222538.520C91F000FF@smtp.kernel.org> Precedence: bulk X-Mailing-List: mm-commits@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The patch titled Subject: writeback: report a Tasks-RCU quiescent state per cgwb drain pass has been added to the -mm mm-hotfixes-unstable branch. Its filename is writeback-report-a-tasks-rcu-quiescent-state-per-cgwb-drain-pass.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/writeback-report-a-tasks-rcu-quiescent-state-per-cgwb-drain-pass.patch This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via various branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there most days ------------------------------------------------------ From: Josef Bacik Subject: writeback: report a Tasks-RCU quiescent state per cgwb drain pass Date: Wed, 09 Sep 2026 18:01:07 +0000 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 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 Cc: David Hildenbrand Cc: Dennis Zhou Cc: Liam R. Howlett Cc: Lorenzo Stoakes Cc: Matthew Wilcox (Oracle) Cc: Michal Hocko Cc: Mike Rapoport Cc: "Paul E . McKenney" Cc: Suren Baghdasaryan Cc: Vlastimil Babka Cc: Signed-off-by: Andrew Morton --- mm/backing-dev.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) --- a/mm/backing-dev.c~writeback-report-a-tasks-rcu-quiescent-state-per-cgwb-drain-pass +++ a/mm/backing-dev.c @@ -910,8 +910,9 @@ 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); _ Patches currently in -mm which might be from josef@toxicpanda.com are writeback-report-a-tasks-rcu-quiescent-state-per-cgwb-drain-pass.patch