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 3DB784A99DC for ; Thu, 3 Sep 2026 12:45:14 +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=1788439515; cv=none; b=jtMpp5mvFXPv+uaAm7xAW/T7ymKdePyshjON9VFXA2l5BWC2Yu77NOLkOgzPGwCsXqCBGqknQMDVH8kSChn9m4D+VNAPSiz6IxNBOXGlf5YGmcuVUB3cDa+7weVcW3DObdI+q1zRJJEGipv2q3Le4/ljrtMxB5LQTAk1wSH8TsQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788439515; c=relaxed/simple; bh=DzxE4YmkzpDr/TZ4i27NXnoGNxYX0+e94d8Eq3S88SY=; h=Subject:To:Cc:From:Date:Message-ID:MIME-Version:Content-Type; b=Skvm0CWB3uz85OaqrT676F1fhnXuEvLr/ZvlXixLt8WpWCRcvkiJApgKEKzEQkzNadRTVwC7D/23r1R2OHKaOyCvPA9bgRnANq9jUQ+MRSiaXUsEO+QvloQ5522+BRYQ47CcTzQPsWU7AZG+887AxQRwGRxZV3VChvTDgQi/x6E= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=TYj+KytH; 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="TYj+KytH" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 613FC1F000E9; Thu, 3 Sep 2026 12:45:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1788439513; bh=pOm6ds8aCqPubv6tjsGtwod7IMDniQbnW6RHG+86MZY=; h=Subject:To:Cc:From:Date; b=TYj+KytHExVL/FnUNgIjZFzSnhQR+5tM1KjKsW+6b2Y93XT8aanu0Cb0c1TkyokaH jIRRMnpkT5Spvmg6FjMNgqhbrD8JVNaYKFmtA59mnrGyZ42dnVyyPIMp14WFD9xrK/ s7KTf1CfmsrMQMaTMELlmE0Zo2vPJ0AeGfAvCKfY= Subject: FAILED: patch "[PATCH] mm/kmemleak: avoid soft lockup when scanning task stacks" failed to apply to 6.6-stable tree To: leitao@debian.org,akpm@linux-foundation.org,cai@lca.pw,catalin.marinas@arm.com,dave@stgolabs.net,lance.yang@linux.dev,oleg@redhat.com,sj@kernel.org,stable@vger.kernel.org Cc: From: Date: Thu, 03 Sep 2026 12:47:32 +0200 Message-ID: <2026090332-ivy-cornflake-86f3@gregkh> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=ANSI_X3.4-1968 Content-Transfer-Encoding: 8bit The patch below does not apply to the 6.6-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to . To reproduce the conflict and resubmit, you may use the following commands: git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y git checkout FETCH_HEAD git cherry-pick -x 5d10d4e19e6daa487f0cd0ea6cba472325de92f9 # git commit -s git send-email --to '' --in-reply-to '2026090332-ivy-cornflake-86f3@gregkh' --subject-prefix 'PATCH 6.6.y' 'HEAD^..' Possible dependencies: thanks, greg k-h ------------------ original commit in Linus's tree ------------------ >From 5d10d4e19e6daa487f0cd0ea6cba472325de92f9 Mon Sep 17 00:00:00 2001 From: Breno Leitao Date: Mon, 15 Jun 2026 10:49:06 -0700 Subject: [PATCH] mm/kmemleak: avoid soft lockup when scanning task stacks Patch series "mm/kmemleak: avoid soft lockup when scanning task", v3. kmemleak_scan() scans every task stack under one rcu_read_lock() with no reschedule point, which can trip the soft lockup watchdog on hosts with very many threads. That prints the following message, depending on the workload+host configuration: watchdog: BUG: soft lockup - CPU#35 stuck for 22s! [kmemleak:537] scan_block kmemleak_scan kmemleak_scan_thread kthread Patch 1 walks the tasks with find_ge_pid() so the scan reschedules between tasks Patches 2-3 let the scan loops stop early once a scan is interrupted. This patch (of 3): kmemleak_scan() walks every thread and scans its kernel stack under a single rcu_read_lock() with no reschedule point. On a host with very many threads -- amplified by KASAN/lockdep in debug builds -- this loop can hog a CPU long enough to trip the soft lockup watchdog: watchdog: BUG: soft lockup - CPU#35 stuck for 22s! [kmemleak:537] scan_block kmemleak_scan kmemleak_scan_thread kthread A cond_resched() cannot be added directly: the loop runs inside an RCU read-side critical section. Walk the tasks one PID at a time with find_ge_pid(), taking the RCU read lock only to look up and pin each task. The stack is then scanned with no lock held, so cond_resched() runs between tasks and the scan stops early on scan_should_stop(). This follows the next_tgid()/task_seq_get_next() iteration pattern and keeps each RCU critical section short. Link: https://lore.kernel.org/20260615-kmemleak-stack-resched-v3-0-acecd7d7fd92@debian.org Link: https://lore.kernel.org/20260615-kmemleak-stack-resched-v3-1-acecd7d7fd92@debian.org Fixes: c4b28963fd79 ("mm/kmemleak: rely on rcu for task stack scanning") Signed-off-by: Breno Leitao Reviewed-by: Catalin Marinas Reviewed-by: Davidlohr Bueso Reviewed-by: Lance Yang Reviewed-by: Oleg Nesterov Cc: Qian Cai Cc: SeongJae Park Cc: Signed-off-by: Andrew Morton diff --git a/mm/kmemleak.c b/mm/kmemleak.c index e196f53f9b46..16b72cead07d 100644 --- a/mm/kmemleak.c +++ b/mm/kmemleak.c @@ -1696,6 +1696,42 @@ static void kmemleak_cond_resched(struct kmemleak_object *object) put_object(object); } +/* + * Scan all task kernel stacks, rescheduling between tasks. Each task is looked + * up and pinned within its own RCU read-side section, so no lock is held across + * the scan and the walk cannot trip the soft lockup watchdog. + */ +static void kmemleak_scan_task_stacks(void) +{ + struct pid *pid; + int nr = 1; + + do { + struct task_struct *p = NULL; + + rcu_read_lock(); + pid = find_ge_pid(nr, &init_pid_ns); + if (pid) { + nr = pid_nr(pid) + 1; + p = pid_task(pid, PIDTYPE_PID); + if (p) + get_task_struct(p); + } + rcu_read_unlock(); + + if (p) { + void *stack = try_get_task_stack(p); + + if (stack) { + scan_block(stack, stack + THREAD_SIZE, NULL); + put_task_stack(p); + } + put_task_struct(p); + } + cond_resched(); + } while (pid && !scan_should_stop()); +} + /* * Print one leak inline. The hex dump is gated on OBJECT_ALLOCATED so it * does not touch user memory that was freed concurrently; the rest of the @@ -1885,19 +1921,8 @@ static void kmemleak_scan(void) /* * Scanning the task stacks (may introduce false negatives). */ - if (kmemleak_stack_scan) { - struct task_struct *p, *g; - - rcu_read_lock(); - for_each_process_thread(g, p) { - void *stack = try_get_task_stack(p); - if (stack) { - scan_block(stack, stack + THREAD_SIZE, NULL); - put_task_stack(p); - } - } - rcu_read_unlock(); - } + if (kmemleak_stack_scan) + kmemleak_scan_task_stacks(); /* * Scan the objects already referenced from the sections scanned