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 2B7934B1B3D for ; Thu, 3 Sep 2026 18:27:02 +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=1788460026; cv=none; b=Rf/JrCoKj/CkTdCBseuQgPgIxfsYDzAbQxpOCMLc/dd/slhWjHV1DrwVilCxSD5aUvjSq5+565394FbVyDDWlZSKy3/4WU5AzcuG6U3Vyo/ij3JMw048pqOs2Kp3PZtL77jMWS5k3nNONT4Ho46nN/RhAQ+k6SKwaB9Nd7yMQhQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788460026; c=relaxed/simple; bh=LJat/t/fPE8aC+azoEyEZPUrrSfmrmHDnBamhvIjqSk=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=oIHupO3X4Wn1ry9JP9T9FAmKOu/vhwPDcUVrUBCT/Bjnl+wQYYdloY9yNC6iR1VLg+zJF/ccvoUlMW1e91BG1KqerLVslACzSDOBIt/MLjqFYK2xC6RGndmPeI6jEaXUArL2/tZvxXgq3YDUdyPBQ8HC5k4fW9jxqJQoiEEk/nM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=dbTFCYt6; 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="dbTFCYt6" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 844781F00A3E; Thu, 3 Sep 2026 18:26:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788460020; bh=htcEWFbFmWPCLq1zKbNwVsJYH8c7DyvbszRGWfo+R8Y=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=dbTFCYt63PV+YV4o/UBxkJV38L2tJYtj8LeJTrvLtDIJCZ2p1W5Yyr3XNBhn+WUSV C7Xp8MOdR6Bb3+TeHngWykYSZNK2sg9zGZSH4OSDA97zxihwH9lw4M8DY/6D6zPPbO xmD6mvg5A3s7QeDqsA7FjhHdWfeafYDlPJDONhH812oLt9V79zJDpgSrNkLuO6wP5m Ah697xnGrXP3rNOWci+zTIS77WamZbKh5DnPGqL6HTo4DHYCIyTN9WjGpXgh1+nwnI TxHJ15glgI6CHJ++5ptWkxeeKteG0AEOG3cw2eOQurKKuFPtOj/iKDcMr/TZZdU/6c 3XAVWmsdsMO1Q== 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 6.18.y 1/2] mm/kmemleak: avoid soft lockup when scanning task stacks Date: Thu, 3 Sep 2026 14:26:57 -0400 Message-ID: <20260903182658.1452185-1-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <2026090344-parted-gnarly-5282@gregkh> References: <2026090344-parted-gnarly-5282@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 | 91 ++++++++++++++++++++++++++++++++++++++------------- 1 file changed, 68 insertions(+), 23 deletions(-) diff --git a/mm/kmemleak.c b/mm/kmemleak.c index 415b154fdf38e..5109a9f84ea50 100644 --- a/mm/kmemleak.c +++ b/mm/kmemleak.c @@ -1518,22 +1518,25 @@ static int scan_should_stop(void) /* * Scan a memory block (exclusive range) for valid pointers and add those - * found to the gray list. + * found to the gray list. Return non-zero if the scan was interrupted. */ -static void scan_block(void *_start, void *_end, - struct kmemleak_object *scanned) +static int scan_block(void *_start, void *_end, + struct kmemleak_object *scanned) { unsigned long *ptr; unsigned long *start = PTR_ALIGN(_start, BYTES_PER_POINTER); unsigned long *end = _end - (BYTES_PER_POINTER - 1); unsigned long flags; + int stop = 0; raw_spin_lock_irqsave(&kmemleak_lock, flags); for (ptr = start; ptr < end; ptr++) { unsigned long pointer; - if (scan_should_stop()) + if (scan_should_stop()) { + stop = 1; break; + } kasan_disable_current(); pointer = *(unsigned long *)kasan_reset_tag((void *)ptr); @@ -1543,22 +1546,28 @@ static void scan_block(void *_start, void *_end, pointer_update_refs(scanned, pointer, OBJECT_PERCPU); } raw_spin_unlock_irqrestore(&kmemleak_lock, flags); + + return stop; } /* * Scan a large memory block in MAX_SCAN_SIZE chunks to reduce the latency. + * Return non-zero if the scan was interrupted. */ #ifdef CONFIG_SMP -static void scan_large_block(void *start, void *end) +static int scan_large_block(void *start, void *end) { void *next; while (start < end) { next = min(start + MAX_SCAN_SIZE, end); - scan_block(start, next, NULL); + if (scan_block(start, next, NULL)) + return 1; start = next; cond_resched(); } + + return 0; } #endif @@ -1689,6 +1698,43 @@ 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; + int stop = 0; + + 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) { + stop = scan_block(stack, stack + THREAD_SIZE, NULL); + put_task_stack(p); + } + put_task_struct(p); + } + cond_resched(); + } while (pid && !stop); +} + /* * Scan data sections and all the referenced memory blocks allocated via the * kernel's standard allocators. This function must be called with the @@ -1700,6 +1746,7 @@ static void kmemleak_scan(void) struct zone *zone; int __maybe_unused i; int new_leaks = 0; + int stop = 0; jiffies_last_scan = jiffies; @@ -1743,9 +1790,11 @@ static void kmemleak_scan(void) #ifdef CONFIG_SMP /* per-cpu sections scanning */ - for_each_possible_cpu(i) - scan_large_block(__per_cpu_start + per_cpu_offset(i), - __per_cpu_end + per_cpu_offset(i)); + for_each_possible_cpu(i) { + if (scan_large_block(__per_cpu_start + per_cpu_offset(i), + __per_cpu_end + per_cpu_offset(i))) + goto scan_gray; + } #endif /* @@ -1772,32 +1821,28 @@ static void kmemleak_scan(void) /* only scan if page is in use */ if (page_count(page) == 0) continue; - scan_block(page, page + 1, NULL); + stop = scan_block(page, page + 1, NULL); + if (stop) + break; } + if (stop) + break; } put_online_mems(); + if (stop) + goto scan_gray; /* * 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 * above. */ +scan_gray: scan_gray_list(); /* -- 2.53.0