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 9BC5F4A33E1 for ; Thu, 3 Sep 2026 17:49:12 +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=1788457754; cv=none; b=O49I5c8nwGVNuwH/Epn3VHuAP0BXlScW8gbUNJTNRvXX+WWDX0VTUN6t4eJlyiJUtp3FYLWkM89y0JMvNclO8Oh0jin2Ag/3G4xoWY8+GP8vjYxuoq7v90+1wDfD7dLAhqQXOSWZMKMR9zkayduMy0Ezc8AHWFyCwJDZqyQ+wbM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788457754; c=relaxed/simple; bh=1xDBeA2iYZJQUcJRCa2RlY1bRxHNsFVV7IXhef3mbLc=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=CT4KTzrDiHkDjsPNbYOfqFpyd39++BJwXKCKTB3Xx/5GRt7ms9GlrnmIgmeDB4Xd7qW163OqQb23/qcyT0+p2FKjtoI6BL6cC+7yzZ/4S05bXJ2jcULPvOFD3HqMOhvoOiKaWbb5QiJwYZNUbXEL0L5h+M4dxYb4Mw6UUNRIq/w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=UTMAKyKD; 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="UTMAKyKD" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 282F01F000E9; Thu, 3 Sep 2026 17:49:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788457752; bh=JIfN+SE+Rx6dtYA8yTdwHKTwUbm3cvUS/lUBUMR/JKk=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=UTMAKyKD+ebf4GezeYZ6mnsntjIYgx3f6hq8SScw8FVBEclif63flk+KfTuItw8HM /179LQolbZStthqLtVazttcV5Vm8qerC4woT4Qkg5V8V0Weea2nK63hJXxMX1XjiSR XJVhckoskoNO5UlwHn8LSmUIH4/cWEJWQCsBX4Fzw+KtbwVVLPhhVcFQvEzFEv8NPA cqf2DAZMwI2i111VqXYlA0TzYycCurP4RkBl4tUT1g1QNrDitZnpUMz3FEVIAORMz0 pu2vMlIeitBAE8rfpRzK3J3xR0sMBHmNSbD2Uakk4clZmD+7oBauGgog+efHzBvf5W KDvo1tpHMhSmw== From: Sasha Levin To: stable@vger.kernel.org Cc: Breno Leitao , Catalin Marinas , Davidlohr Bueso , Lance Yang , Oleg Nesterov , Qian Cai , SeongJae Park , Andrew Morton , Sasha Levin Subject: [PATCH 7.2.y 1/3] mm/kmemleak: avoid soft lockup when scanning task stacks Date: Thu, 3 Sep 2026 13:49:06 -0400 Message-ID: <20260903174908.1268466-1-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <2026090344-tightly-handiwork-36ab@gregkh> References: <2026090344-tightly-handiwork-36ab@gregkh> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Breno Leitao [ Upstream commit 5d10d4e19e6daa487f0cd0ea6cba472325de92f9 ] 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 Stable-dep-of: 3541a2b06ecd ("mm/kmemleak: report RCU-tasks quiescent states during the scan") Signed-off-by: Sasha Levin --- mm/kmemleak.c | 51 ++++++++++++++++++++++++++++++++++++++------------- 1 file changed, 38 insertions(+), 13 deletions(-) diff --git a/mm/kmemleak.c b/mm/kmemleak.c index e196f53f9b462..16b72cead07d2 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 -- 2.53.0