From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id CFE65CD98C7 for ; Fri, 12 Jun 2026 03:16:17 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 167286B0005; Thu, 11 Jun 2026 23:16:17 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 117896B0092; Thu, 11 Jun 2026 23:16:17 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 02D676B0093; Thu, 11 Jun 2026 23:16:16 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id E27F66B0005 for ; Thu, 11 Jun 2026 23:16:16 -0400 (EDT) Received: from smtpin12.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay01.hostedemail.com (Postfix) with ESMTP id A6A9D1C2286 for ; Fri, 12 Jun 2026 03:16:16 +0000 (UTC) X-FDA: 84869797152.12.6E56232 Received: from out-186.mta0.migadu.com (out-186.mta0.migadu.com [91.218.175.186]) by imf26.hostedemail.com (Postfix) with ESMTP id E9EC3140004 for ; Fri, 12 Jun 2026 03:16:14 +0000 (UTC) Authentication-Results: imf26.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=Xx9XYhMp; spf=pass (imf26.hostedemail.com: domain of lance.yang@linux.dev designates 91.218.175.186 as permitted sender) smtp.mailfrom=lance.yang@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1781234175; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=v4xgDk90vxqb8XarhjHuqrDXk08OYdv+zHxOLhLCQ3Y=; b=oImB/SfMZ9sWk0fRXqCYyFP4ElBgMytvVgUKRQgiY4C9mr682ZQiulOebVFYaOKTiAta/R IsRoU9pMFKwptTG4LxH8kDBu67m3fsh9EVpvEIyL0Zbl+PK/q3+WFvugmJgvPUYUQ5pf0z SvL9v8o3qttjQFBjaUD+x15+gtOgWc4= ARC-Authentication-Results: i=1; imf26.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=Xx9XYhMp; spf=pass (imf26.hostedemail.com: domain of lance.yang@linux.dev designates 91.218.175.186 as permitted sender) smtp.mailfrom=lance.yang@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1781234175; b=Dl1pgIjrTyxCduBwP/AhgBLOSwlxDSjOevO2EFov+NK6oTym+YrUPH7N2CN72YeN+X/s8o QfjsJ5hBCvMW2K+eC9cCpHg+4noUT+dkrq/K2ccFC91+FOjfCUFiVtF/6q63u3xX1so/7t I5yJIuo065VnfI2qvdV3shC/Cfk2R9o= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1781234173; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=v4xgDk90vxqb8XarhjHuqrDXk08OYdv+zHxOLhLCQ3Y=; b=Xx9XYhMpwYMwxA5cUtqpoJgWsfjFRt+255joY5b8hbqfOz6vyjR2t29+5mxL+FlTpSGAiB aXCNeXlZh6gzcp7udtmxDXpCtf5XgqS4V7SPPLO8Chj+ztarqcZgZrERtYNIuUn3q6ArUC G883H5IlHOXLXqKP+tBwoW+J0EmTMkg= From: Lance Yang To: leitao@debian.org Cc: catalin.marinas@arm.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, sj@kernel.org, Lance Yang Subject: Re: [PATCH RFC] mm/kmemleak: avoid soft lockup when scanning task stacks Date: Fri, 12 Jun 2026 11:16:05 +0800 Message-Id: <20260612031605.58235-1-lance.yang@linux.dev> In-Reply-To: <20260611-kmemleak-stack-resched-v1-1-d6248ade5f4a@debian.org> References: <20260611-kmemleak-stack-resched-v1-1-d6248ade5f4a@debian.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: E9EC3140004 X-Stat-Signature: pfjagpjo3u4xptkwp3jwbbc66a5yu6cr X-Rspam-User: X-HE-Tag: 1781234174-94775 X-HE-Meta: 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 Njm0Lkww YxrKDWlrBXHAtM5oyCvjauGEt1DVR7qg8EZ5dA5QaFv1E7zxg+zPoSUP+YFETh2ay3LWCzOZXcBMKMmkHaqRZHXhC5jdBs50sfeWb+yL88eQAhwbn+x/DEGA7AxTtqqTgcJE6y1VzQSm4R3RKWpD6o+N7FNZjRRO5LdRSNuNUuIkrNn5yI63wdpllNyR4ie/bcnFStCGTi2+D0uu7xJJlAC8RiOxdb6WDhZdlRmwWrsE9V9U= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Jun 11, 2026 at 05:45:00AM -0700, Breno Leitao wrote: >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 Neat, good catch! >A cond_resched() cannot be added directly: the loop runs inside an RCU >read-side critical section. > >Split the scan in two parts: > >1) get the list of tasks (with RCU read lock) in an array >2) run scan_block() for the tasks (with cond_reschd()). > >Is it a sane approach? Why not use the kernel/hung_task.c pattern here? Seems simpler, with no extra task-array allocation ;) >Signed-off-by: Breno Leitao >--- Could break RCU only when resched is needed. Pin the current cursors, drop RCU, cond_resched(), take RCU again, and continue only if the cursors are still alive ;) If either cursor died while RCU was droped, stopping this scan round should be fine, IMHO. ---8<--- diff --git a/mm/kmemleak.c b/mm/kmemleak.c index 7c7ba17ce7af..1062d9545054 100644 --- a/mm/kmemleak.c +++ b/mm/kmemleak.c @@ -1695,6 +1695,26 @@ static void kmemleak_cond_resched(struct kmemleak_object *object) put_object(object); } +static bool kmemleak_stack_scan_break(struct task_struct *g, + struct task_struct *p) +{ + bool can_cont; + + get_task_struct(g); + get_task_struct(p); + + rcu_read_unlock(); + cond_resched(); + rcu_read_lock(); + + can_cont = pid_alive(g) && pid_alive(p); + + put_task_struct(p); + put_task_struct(g); + + return can_cont; +} + /* * 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 @@ -1894,7 +1914,10 @@ static void kmemleak_scan(void) scan_block(stack, stack + THREAD_SIZE, NULL); put_task_stack(p); } + if (need_resched() && !kmemleak_stack_scan_break(g, p)) + goto unlock; } +unlock: rcu_read_unlock(); } --- Not tested, though, feel free to grab it if looks sane :) [...] Cheers, Lance