From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (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 0F44A20468F for ; Sat, 25 Jan 2025 01:55:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737770110; cv=none; b=FfjixUkWgD/5zJ1PQ3ye/+vEswAMICnNre5Qcfw15KsDDUiqaEBgHaqFpeoP3KZ4v5vcBIj7z7mSDDrHRFYynK3Rvu8B54ZrgcK5elD3zOfLbSqK5/n0//CIM7Vf6YXnIDTESFfGgCHQ9rhLxPm6M12l/OtTNFYu0SG3C/dFdtw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737770110; c=relaxed/simple; bh=1SKBaYq6MrcXbMp5TSQ4b1mjUvW+9Fe5VKe3IHjht2U=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=dCpARFJZx5jklGFynlsp47X8GfN+BUzP9IsWoTnEmZV0ZXBD8SH40ilYk0ApRrXmKLmBeFGhYtKzOxkENCIhTrgiLa8Kmp0f3yzyGwSIl+LsPxPlslj40L0r6brduTr/DIE7V8AFJxW54HaxHCfAYFBw/M6WnP9gTTLjS1A7G2I= 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=RXJv4/cT; arc=none smtp.client-ip=170.10.129.124 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="RXJv4/cT" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1737770107; 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=YfBI69kTG1U0a5VnaZ+DhqzZrCV3aR94+OU9hGCmaV0=; b=RXJv4/cTyYV+z+LvPughvRO5+AoRqZ4sfqOZ6lQMbDyqYs59c4yXoafMSISfu5uqKjjE82 Z8KGk5g9OrusRLq1HxRfqVnafjb8zp4G4nnB80sSqFeGJ9tZBDDRwHABjKmCgOZr327bVw FPBaq3I7SCl6UsT6b57CWt8t0a264rU= Received: from mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-115-_d7_un-aOrywPzPwgKZfJw-1; Fri, 24 Jan 2025 20:55:03 -0500 X-MC-Unique: _d7_un-aOrywPzPwgKZfJw-1 X-Mimecast-MFC-AGG-ID: _d7_un-aOrywPzPwgKZfJw 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-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 2E52C19560AF; Sat, 25 Jan 2025 01:55:02 +0000 (UTC) Received: from llong-thinkpadp16vgen1.westford.csb (unknown [10.22.88.114]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id D3B681800348; Sat, 25 Jan 2025 01:54:59 +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 v2 2/2] clocksource: Use get_random_bytes() in clocksource_verify_choose_cpus() Date: Fri, 24 Jan 2025 20:54:42 -0500 Message-ID: <20250125015442.3740588-2-longman@redhat.com> In-Reply-To: <20250125015442.3740588-1-longman@redhat.com> References: <20250125015442.3740588-1-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. If crng_ready() is true by the time get_random_u32() is called, The batched_entropy_32 local lock will be acquired. In PREEMPT_RT kernel, it is a rtmutex and we can't acquire it with preemption disabled. Fix this problem by using the less random get_random_bytes() function which will not take any lock. In fact, it has the same random-ness as get_random_u32_below() when crng_ready() is false. Fixes: 7560c02bdffb ("clocksource: Check per-CPU clock synchronization when marked unstable") Signed-off-by: Waiman Long --- kernel/time/clocksource.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index 77d9566d3aa6..659c4b79119c 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -340,9 +340,13 @@ static void clocksource_verify_choose_cpus(void) * and no replacement CPU is selected. This gracefully handles * situations where verify_n_cpus is greater than the number of * CPUs that are currently online. + * + * The get_random_bytes() is used here to avoid taking lock with + * preemption disabled. */ for (i = 1; i < n; i++) { - cpu = get_random_u32_below(nr_cpu_ids); + get_random_bytes(&cpu, sizeof(cpu)); + cpu %= nr_cpu_ids; cpu = cpumask_next(cpu - 1, cpu_online_mask); if (cpu >= nr_cpu_ids) cpu = cpumask_first(cpu_online_mask); -- 2.47.1