From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 E3EF61E25FA for ; Wed, 29 Jan 2025 22:40:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738190429; cv=none; b=SyWWL6hPstjd1PixjjXuMpepqaeGeoGv3+UKvfNElpBI19t8IKHSzwFczmeKxqKVf85e63kYWtSXMttXcVVS8WK6MABBQKI4kBETvxMv8tQnbGG6jj+KuSRI2M6q3098Jd8UMjPc+IiMqYSUAEI9RL+Pq1cFudDdNIxc0Mvr/7c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738190429; c=relaxed/simple; bh=M0n5hsxEGBIHAPMTtB8uz8dBIRCZLdmTK1sUi6haSOA=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=MqTi2lx+y0GXSRJshgOKOWawP3pgZ1FndGQqq1NxBn95/XGwSxEDXQlPeS/MTAskNXE5vXYWNaCu6yIAz8JmAUb3tUou/cBFfhiNLKQnei1K2U2LYBvKmD+pOXldZfGhemdNZbvj/hJQjJo4QW9l9WAy4p2l4H8ypBh4hAc1Sqo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=LRsVaGWx; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="LRsVaGWx" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1738190427; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=HKxImgEo+uYTNeeJc5s+v8Bn7hVMZT0zwqE6/m/ojLU=; b=LRsVaGWxKVe3GR1dyFoR9t2iz/neN3n6P8IY8Xb9nWjbvRmWtKlPT57O//XxYR/M3yQnxd w1ZeF9hic+vMugUydzzlE3NKhIUd7ax7PG6++LAgy76dTjNP/z2iScYI8xW54X0NWQPDHz Fkq7Ut+uF+JkHhwD59LCiLfuLL2jQbs= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-674-owF4nglIMk6hd6dZDCrhGg-1; Wed, 29 Jan 2025 17:40:23 -0500 X-MC-Unique: owF4nglIMk6hd6dZDCrhGg-1 X-Mimecast-MFC-AGG-ID: owF4nglIMk6hd6dZDCrhGg Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 667E81801F1E; Wed, 29 Jan 2025 22:40:21 +0000 (UTC) Received: from llong-thinkpadp16vgen1.westford.csb (unknown [10.22.64.23]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id F25971800267; Wed, 29 Jan 2025 22:40:18 +0000 (UTC) From: Waiman Long To: John Stultz , Thomas Gleixner , Stephen Boyd , Feng Tang , "Paul E. McKenney" , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt Cc: linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, Waiman Long Subject: [PATCH v3 2/2] clocksource: Use get_random_bytes() in clocksource_verify_choose_cpus() Date: Wed, 29 Jan 2025 17:40:01 -0500 Message-ID: <20250129224001.430506-2-longman@redhat.com> In-Reply-To: <20250129224001.430506-1-longman@redhat.com> References: <20250129224001.430506-1-longman@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 The following bug report happened in a PREEMPT_RT kernel. [ 30.957705] BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48 [ 30.957711] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 2012, name: kwatchdog [ 30.962673] preempt_count: 1, expected: 0 [ 30.962676] RCU nest depth: 0, expected: 0 [ 30.962680] 3 locks held by kwatchdog/2012: [ 30.962684] #0: ffffffff8af2da60 (clocksource_mutex){+.+.}-{3:3}, at: clocksource_watchdog_kthread+0x13/0x50 [ 30.967703] #1: ffffffff8aa8d4d0 (cpu_hotplug_lock){++++}-{0:0}, at: clocksource_verify_percpu.part.0+0x5c/0x330 [ 30.972774] #2: ffff9fe02f5f33e0 ((batched_entropy_u32.lock)){+.+.}-{2:2}, at: get_random_u32+0x4f/0x110 [ 30.977827] Preemption disabled at: [ 30.977830] [] clocksource_verify_percpu.part.0+0x66/0x330 [ 30.982837] CPU: 33 PID: 2012 Comm: kwatchdog Not tainted 5.14.0-503.23.1.el9_5.x86_64+rt-debug #1 [ 30.982843] Hardware name: HPE ProLiant DL385 Gen10 Plus/ProLiant DL385 Gen10 Plus, BIOS A42 04/29/2021 [ 30.982846] Call Trace: [ 30.982850] [ 30.983821] dump_stack_lvl+0x57/0x81 [ 30.983821] __might_resched.cold+0xf4/0x12f [ 30.983824] rt_spin_lock+0x4c/0x100 [ 30.988833] get_random_u32+0x4f/0x110 [ 30.988833] clocksource_verify_choose_cpus+0xab/0x1a0 [ 30.988833] clocksource_verify_percpu.part.0+0x6b/0x330 [ 30.993894] __clocksource_watchdog_kthread+0x193/0x1a0 [ 30.993898] clocksource_watchdog_kthread+0x18/0x50 [ 30.993898] kthread+0x114/0x140 [ 30.993898] ret_from_fork+0x2c/0x50 [ 31.002864] It is due to the fact that get_random_u32() is called in clocksource_verify_choose_cpus() with preemption disabled. The batched_entropy_32 local lock and/or the base_crng.lock spinlock will be acquired. In PREEMPT_RT kernel, they are rtmutexes and the above warning will be printed if the fast path fails because of contention. Fix this problem by moving the clocksource_verify_choose_cpus() call before preempt_disable() while moving the part that needs preemption to be disabled out into a new clocksource_verify_fixup_cpus() helper that is called after preempt_disable(). In that way, the get_random_u32() function will now be called with preemption enabled. Fixes: 7560c02bdffb ("clocksource: Check per-CPU clock synchronization when marked unstable") Signed-off-by: Waiman Long --- kernel/time/clocksource.c | 42 ++++++++++++++++++++++++++------------- 1 file changed, 28 insertions(+), 14 deletions(-) diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index 77d9566d3aa6..08d6ac3a795f 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -313,23 +313,14 @@ static void clocksource_verify_choose_cpus(void) if (n < 0) { /* Check all of the CPUs. */ cpumask_copy(&cpus_chosen, cpu_online_mask); - cpumask_clear_cpu(smp_processor_id(), &cpus_chosen); return; } /* If no checking desired, or no other CPU to check, leave. */ cpumask_clear(&cpus_chosen); - if (n == 0 || num_online_cpus() <= 1) + if (n <= 1 || num_online_cpus() <= 1) return; - /* Make sure to select at least one CPU other than the current CPU. */ - cpu = cpumask_first(cpu_online_mask); - if (cpu == smp_processor_id()) - cpu = cpumask_next(cpu, cpu_online_mask); - if (WARN_ON_ONCE(cpu >= nr_cpu_ids)) - return; - cpumask_set_cpu(cpu, &cpus_chosen); - /* Force a sane value for the boot parameter. */ if (n > nr_cpu_ids) n = nr_cpu_ids; @@ -341,7 +332,7 @@ static void clocksource_verify_choose_cpus(void) * situations where verify_n_cpus is greater than the number of * CPUs that are currently online. */ - for (i = 1; i < n; i++) { + for (i = 0; i < n; i++) { cpu = get_random_u32_below(nr_cpu_ids); cpu = cpumask_next(cpu - 1, cpu_online_mask); if (cpu >= nr_cpu_ids) @@ -349,9 +340,32 @@ static void clocksource_verify_choose_cpus(void) if (!WARN_ON_ONCE(cpu >= nr_cpu_ids)) cpumask_set_cpu(cpu, &cpus_chosen); } +} + +/* + * Return: true if success, false if not + */ +static bool clocksource_verify_fixup_cpus(void) +{ + int testcpu = smp_processor_id(); + int cpu, n = verify_n_cpus; + bool ret; /* Don't verify ourselves. */ - cpumask_clear_cpu(smp_processor_id(), &cpus_chosen); + cpumask_clear_cpu(testcpu, &cpus_chosen); + + ret = !cpumask_empty(&cpus_chosen); + if (!ret && (n != 0) && (num_online_cpus() > 1)) { + /* Make sure to select at least one CPU other than the current CPU. */ + cpu = cpumask_first(cpu_online_mask); + if (cpu == testcpu) + cpu = cpumask_next(cpu, cpu_online_mask); + if (WARN_ON_ONCE(cpu >= nr_cpu_ids)) + return false; + cpumask_set_cpu(cpu, &cpus_chosen); + return true; + } + return ret; } static void clocksource_verify_one_cpu(void *csin) @@ -373,9 +387,9 @@ void clocksource_verify_percpu(struct clocksource *cs) cpumask_clear(&cpus_ahead); cpumask_clear(&cpus_behind); cpus_read_lock(); - preempt_disable(); clocksource_verify_choose_cpus(); - if (cpumask_empty(&cpus_chosen)) { + preempt_disable(); + if (!clocksource_verify_fixup_cpus()) { preempt_enable(); cpus_read_unlock(); pr_warn("Not enough CPUs to check clocksource '%s'.\n", cs->name); -- 2.48.1