From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from stravinsky.debian.org (stravinsky.debian.org [82.195.75.108]) (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 9E5A73F4111 for ; Thu, 11 Jun 2026 12:45:15 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=82.195.75.108 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781181917; cv=none; b=oQZucBv7OpgTpS8dl/em+zI+xvQF5ypZ3qDpgMVB0XzYJIULzujO8Pli9WXpPBy2liDslyBztT0D4tU91e+WG3XdhIoBmSdKBS+QwxVJ2ARRFDXwy5TTK9zlMnH35OgX8KrCaWoiVxL9VarfeI73vaJNmiT9xvfbJF/5ZpoXN0M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781181917; c=relaxed/simple; bh=Xzjey3oU6Gako3r0mVlv6us2QUMOKtJPCFgaQ6L1zU0=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:To:Cc; b=NEs1kv5ogVzrQgvn5gJpIc4r3JcS6AYoQiASAvcOjRegmXRRHZAULLthg9UCseihZLlWtIQHereSbgb/HytQZGB1NQ2RINzHD5viWaAF3fSj8JPjD5ZvMIyYAe+RJm+8Sideb2sf7IVc8kksYkcX1Bs1bsHXqjBgAk5AwG9P73s= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=debian.org; spf=pass smtp.mailfrom=debian.org; dkim=pass (2048-bit key) header.d=debian.org header.i=@debian.org header.b=s+vs5POf; arc=none smtp.client-ip=82.195.75.108 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=debian.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=debian.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=debian.org header.i=@debian.org header.b="s+vs5POf" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=debian.org; s=smtpauto.stravinsky; h=X-Debian-User:Cc:To:Message-Id: Content-Transfer-Encoding:Content-Type:MIME-Version:Subject:Date:From: Reply-To:Content-ID:Content-Description:In-Reply-To:References; bh=ANkUBUW0AiqcdqhZCQaCikPj4wJ9n2miiY8ADVD63kM=; b=s+vs5POfS1cOXulTEXiW7K8ehr zY6a8YjV0y/4Ls4zFdLGBCU0ZuCY/5PZRfLopDSytWoqYoeHV74MKFJIw7H2vbq9vRJIHhzBTgJmd 7tO7dJ6RM3XD1DZP9qtLQmirM4LK+cSnNqlzez0J2t9dsYfPUBIkgmvzMPBZx3yHvLRcfmgthQC56 sgfTjFRyYg0T4JhFzyo1SUU9bgyhNFhI2p9Qm0MdfogrDcyWb6lJx4gTTs+Uh1rjXQYtIYmAT9n07 TYkj1gpzSw0r/mU3LheT41vidaSbKyjjSbbxhXkyGtGsNGvBQsof9WPldoEHDduVSSUf2uR9EOGLq KTH6anXQ==; Received: from authenticated-user by stravinsky.debian.org with esmtpsa (TLS1.3:ECDHE_X25519__RSA_PSS_RSAE_SHA256__AES_256_GCM:256) (Exim 4.96) (envelope-from ) id 1wXemU-009xy5-3A; Thu, 11 Jun 2026 12:45:11 +0000 From: Breno Leitao Date: Thu, 11 Jun 2026 05:45:00 -0700 Subject: [PATCH RFC] mm/kmemleak: avoid soft lockup when scanning task stacks Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20260611-kmemleak-stack-resched-v1-1-d6248ade5f4a@debian.org> X-B4-Tracking: v=1; b=H4sIAMutKmoC/x3MQQrCMBAF0KsMf92BJGBbshU8gNviIiRTO8RWy YgIpXcXfAd4O0yaiiHSjiYfNX1uiOQ7Ql7SdhfWgkgILvSu957rKutDUmV7p1y5ieVFCjsvZQj jaUzDjI7wajLr9x9PuF7OuB3HD6uwcCltAAAA X-Change-ID: 20260611-kmemleak-stack-resched-01ed72858a7f To: Catalin Marinas , Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, Breno Leitao X-Mailer: b4 0.14.3 X-Developer-Signature: v=1; a=openpgp-sha256; l=2409; i=leitao@debian.org; h=from:subject:message-id; bh=Xzjey3oU6Gako3r0mVlv6us2QUMOKtJPCFgaQ6L1zU0=; b=owEBbQKS/ZANAwAIATWjk5/8eHdtAcsmYgBqKq3Sd8LpaUaDSfsNMpGN3CKrIplgCKbm0fhUT kFP92mZ4N+JAjMEAAEIAB0WIQSshTmm6PRnAspKQ5s1o5Of/Hh3bQUCaiqt0gAKCRA1o5Of/Hh3 bUjcD/46jQuNP9O2OsG9TaVh86su/9q6InXfWqSnPBF+s/+TQGyVP+mC5ZNKCWKB6iKBCfoiVZz GUyHcW0U39m46s2slTsaH6j47rKDvGLQtUXUNYyVOtT73BuLFp8hxB947/CGvlu/QI2xIEIiy8z ZHkwHstRJGIfRzYnJjLyJzdXEDAilBO09+/z+USPYE79Evi8/I6nBYNd8RXH7Wb9EOOtUeDgyFw i1B+ov5cT54OPzFr8WjheRv+brne0s3c1Szhkf+EwLuZtCjbe1lbkORX+7jLIbO8NrC75xvZBnN 0MUeHwdVSwDa4+mtEQFS0JTB1g118ECqkjjKUWAcbScTTPEPHlC49K+/zTAM07EF763Y3U1RX3J J2aveYR4O0xdyqHG7PK9MbA3nd5y8hED4PFVXdCFsb6yE+4t/lq8GyGtsb64YVoSejUzXbapGxh IJQ6ARWAfz/T34T6z2jQ20cFMViXFEEZ4SxEqOTxFFRo3K+l8ae0yxAa0gGjoEycnGwMsj3+MOg zkfq6OnaeakExjq2BHn/EXgFE9TWajOtfVYPkVBbQ0vAh0PaBYSg6AFBqVv+BXKDq0FS+DzmL8P bIKI6/e6c4Er2OeDudD+U7U4ktfIj3oy8guOkU1joTK/Tf48D5r6/OJnaf+WQ0ULzXlJDnehKt3 /FWKIgixgWp/iBw== X-Developer-Key: i=leitao@debian.org; a=openpgp; fpr=AC8539A6E8F46702CA4A439B35A3939FFC78776D X-Debian-User: leitao 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. 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? Signed-off-by: Breno Leitao --- mm/kmemleak.c | 26 ++++++++++++++++++++++---- 1 file changed, 22 insertions(+), 4 deletions(-) diff --git a/mm/kmemleak.c b/mm/kmemleak.c index 7c7ba17ce7af0..9f8a35ecbb50c 100644 --- a/mm/kmemleak.c +++ b/mm/kmemleak.c @@ -62,6 +62,7 @@ #include #include #include +#include #include #include #include @@ -1885,17 +1886,34 @@ static void kmemleak_scan(void) * Scanning the task stacks (may introduce false negatives). */ if (kmemleak_stack_scan) { - struct task_struct *p, *g; + struct task_struct **tasks, *p, *g; + unsigned int nr = 0, max, i; + max = nr_threads + 64; + tasks = kvmalloc_array(max, sizeof(*tasks), GFP_KERNEL); + + /* Snapshot the threads under RCU */ rcu_read_lock(); for_each_process_thread(g, p) { - void *stack = try_get_task_stack(p); + if (!tasks || nr >= max) + break; + get_task_struct(p); + tasks[nr++] = p; + } + rcu_read_unlock(); + + /* now scan_block for the tasks above with cond_resched() */ + for (i = 0; i < nr; i++) { + void *stack = try_get_task_stack(tasks[i]); + if (stack) { scan_block(stack, stack + THREAD_SIZE, NULL); - put_task_stack(p); + put_task_stack(tasks[i]); } + put_task_struct(tasks[i]); + cond_resched(); } - rcu_read_unlock(); + kvfree(tasks); } /* --- base-commit: abe651837cb394f76d738a7a747322fca3bf17ba change-id: 20260611-kmemleak-stack-resched-01ed72858a7f Best regards, -- Breno Leitao