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 03D8E4FC8EA for ; Thu, 3 Sep 2026 17:48:38 +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=1788457719; cv=none; b=PSDo3kbz+l3mkGTHUVUy8xRVy++iw08z/tY3CxY3p3JQ3WkI3svWP3mR/Bttu51XLJbZvIPLR5zZ57uquYLkK4It40VQVM8Woe8dT8COeOZkVLZMqeXEap8x1K8QNE12AH6xvrvXPlyfs8DGinRlyli5sbG7lwrsx/WyUrDlrtI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788457719; c=relaxed/simple; bh=ezwppTi9JZzvckfv0sxDBpw+aTrhjrvdsRhXb/+FWuw=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=mnWVdl/wtBaNuhaA2EgeP0yB66tPM8WGO34DjLH2xBJ8Of37ll8iuOg50ZUMlSgcaCkja7fwtqOP0Z1mCb8b2jwwcMnbPB/LbVUKpOXA7Ewodg7FY3EKLzpWWnnPJITVOZbUvAKDRjR1NW5X3aNrP2YlwcyZx+SBDw4OoYvx96A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=k4jtl4fF; 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="k4jtl4fF" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DF3DC1F000E9; Thu, 3 Sep 2026 17:48:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788457717; bh=U5HTzNvoIyFFe7xC8j9AsPyVTFfGzVkgbOw2InUv8Fs=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=k4jtl4fFQRV4El3nEsiyyN1NftqEV3VXcqQNIXKvoh2U/oPJFlqLscQbdBM3WSd/J +vbtLgA7flA2o9Gkmk45yx7tkhfdbr70TZ/T5RigOW455wPCWTWmqZIVVMKY/ObqLm x2wHz3qlcTxIYZwZBvdfsxHep9LfFNbnNlKsdJYr3jv70MAu14VChH2ko5RfODO2NF w3uDVeR1W8mLSkIc8INuixzf2pHaa9435KG5FLt2rNosfl+ouStu8GqUgrtYhEBMNK 1MY2k41XgQWbPM9eOe0FhttqE1rEtR9annwkEiswsYzKxFP92VWXg/kjWozje/GCx0 E6Enw7V8Vf/0A== 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 6.1.y 4/4] mm/kmemleak: avoid soft lockup when scanning task stacks Date: Thu, 3 Sep 2026 13:48:32 -0400 Message-ID: <20260903174832.1263842-4-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260903174832.1263842-1-sashal@kernel.org> References: <2026090332-elliptic-contort-6cb3@gregkh> <20260903174832.1263842-1-sashal@kernel.org> 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 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 904b5d396e124..79f5565bb9b92 100644 --- a/mm/kmemleak.c +++ b/mm/kmemleak.c @@ -1486,6 +1486,42 @@ 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()); +} + /* * Scan data sections and all the referenced memory blocks allocated via the * kernel's standard allocators. This function must be called with the @@ -1576,19 +1612,8 @@ 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