From nobody Wed Dec 17 23:27:23 2025 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) (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 AB0931DF686 for ; Tue, 20 Aug 2024 06:30:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=193.142.43.55 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1724135410; cv=none; b=IxHatbAHXL7AdIZi/or6qZCw2dYxKhVoqCJRX5BEoch2bVrhwcsctyWGy/POlWhq42cx+yxa0PSTCxIhYH0p5bcTFQ1qqF+aHXgps6eNZoMzMEg+cyF9ijGNvUyatTrPyj6uXRTF2feXrmcijhI82QPN9pjo2d5r3QITX81T3Fg= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1724135410; c=relaxed/simple; bh=w/0A+I2P64NXX1ai/t0T7MSBWwFyXQZvqP6UKKp/3BQ=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=EXUwEJ4KXw3Y+zL6+bmBCgLcg6H2GRVeH2OWHmg26BXhD/Ps5jPl56BVv6MO5DQ1vyS2JTiZalqDfqU2HoO34PuchsMUvI/x/rndHP8c/rfyBT5uORjRDonVcp9IBuUkh6dfANpJw9NWeARqSMIL6eHluOhLbi00DCv0P4rL/aE= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de; spf=pass smtp.mailfrom=linutronix.de; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=2Vr8xKv4; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=LQqy5mwr; arc=none smtp.client-ip=193.142.43.55 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linutronix.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="2Vr8xKv4"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="LQqy5mwr" From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1724135404; 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=OV1vJv860Ihcd3WShe7ztkuucPJuixldc8WjZwKIQZw=; b=2Vr8xKv4kCs8Bz0Ups8ZooMxF/dCGumZSwA3ACWVtSMIb52iFYYS+He93jZNM6adyqFUHt n5BD9V5gubAEFvmKK1IevhwL2nS+bjdVIhqvWut5NB/eHE3ArYAPOVsrRhLcHsXbsKquY9 AWN1kSD6h3m/dvCvmWxjQMw6FA3Nlvhif0NFDwGU9SvHLPwBo7b9Eq6Vn/lVFgp3iaUJpR cqFml3IF57V4+xj/f3jKZ4wuSvZVdEB2XreQ4mrWmerkGFoedi0x5GucZLt+cEuwVMLXmC wMc2Tt5A5F228rsx7zROzREZsiJ9Syaxr0qg1F5kmkTNe+fePsWVcJoNgCQoxA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1724135404; 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=OV1vJv860Ihcd3WShe7ztkuucPJuixldc8WjZwKIQZw=; b=LQqy5mwrr2t6k/PlP2sjXxQ+2MgvblrIYL8g+n2ZYeu23HjiypJRoCx3d+RZo5j4A3Ijsb SNN7I12aAmJ6/YAg== To: Petr Mladek Cc: Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: [PATCH printk v8 05/35] printk: nbcon: Clarify rules of the owner/waiter matching Date: Tue, 20 Aug 2024 08:35:31 +0206 Message-Id: <20240820063001.36405-6-john.ogness@linutronix.de> In-Reply-To: <20240820063001.36405-1-john.ogness@linutronix.de> References: <20240820063001.36405-1-john.ogness@linutronix.de> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" The functions nbcon_owner_matches() and nbcon_waiter_matches() use a minimal set of data to determine if a context matches. The existing kerneldoc and comments were not clear enough and caused the printk folks to re-prove that the functions are indeed reliable in all cases. Update and expand the explanations so that it is clear that the implementations are sufficient for all cases. Signed-off-by: John Ogness Reviewed-by: Petr Mladek --- kernel/printk/nbcon.c | 56 +++++++++++++++++++++++++++++++++++-------- 1 file changed, 46 insertions(+), 10 deletions(-) diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c index 776746d20fc0..931b8f086902 100644 --- a/kernel/printk/nbcon.c +++ b/kernel/printk/nbcon.c @@ -228,6 +228,13 @@ static int nbcon_context_try_acquire_direct(struct nbc= on_context *ctxt, struct nbcon_state new; =20 do { + /* + * Panic does not imply that the console is owned. However, it + * is critical that non-panic CPUs during panic are unable to + * acquire ownership in order to satisfy the assumptions of + * nbcon_waiter_matches(). In particular, the assumption that + * lower priorities are ignored during panic. + */ if (other_cpu_in_panic()) return -EPERM; =20 @@ -259,18 +266,29 @@ static bool nbcon_waiter_matches(struct nbcon_state *= cur, int expected_prio) /* * The request context is well defined by the @req_prio because: * - * - Only a context with a higher priority can take over the request. + * - Only a context with a priority higher than the owner can become + * a waiter. + * - Only a context with a priority higher than the waiter can + * directly take over the request. * - There are only three priorities. * - Only one CPU is allowed to request PANIC priority. * - Lower priorities are ignored during panic() until reboot. * * As a result, the following scenario is *not* possible: * - * 1. Another context with a higher priority directly takes ownership. - * 2. The higher priority context releases the ownership. - * 3. A lower priority context takes the ownership. - * 4. Another context with the same priority as this context + * 1. This context is currently a waiter. + * 2. Another context with a higher priority than this context + * directly takes ownership. + * 3. The higher priority context releases the ownership. + * 4. Another lower priority context takes the ownership. + * 5. Another context with the same priority as this context * creates a request and starts waiting. + * + * Event #1 implies this context is EMERGENCY. + * Event #2 implies the new context is PANIC. + * Event #3 occurs when panic() has flushed the console. + * Events #4 and #5 are not possible due to the other_cpu_in_panic() + * check in nbcon_context_try_acquire_direct(). */ =20 return (cur->req_prio =3D=3D expected_prio); @@ -578,11 +596,29 @@ static bool nbcon_owner_matches(struct nbcon_state *c= ur, int expected_cpu, int expected_prio) { /* - * Since consoles can only be acquired by higher priorities, - * owning contexts are uniquely identified by @prio. However, - * since contexts can unexpectedly lose ownership, it is - * possible that later another owner appears with the same - * priority. For this reason @cpu is also needed. + * A similar function, nbcon_waiter_matches(), only deals with + * EMERGENCY and PANIC priorities. However, this function must also + * deal with the NORMAL priority, which requires additional checks + * and constraints. + * + * For the case where preemption and interrupts are disabled, it is + * enough to also verify that the owning CPU has not changed. + * + * For the case where preemption or interrupts are enabled, an + * external synchronization method *must* be used. In particular, + * the driver-specific locking mechanism used in device_lock() + * (including disabling migration) should be used. It prevents + * scenarios such as: + * + * 1. [Task A] owns a context with NBCON_PRIO_NORMAL on [CPU X] and + * is scheduled out. + * 2. Another context takes over the lock with NBCON_PRIO_EMERGENCY + * and releases it. + * 3. [Task B] acquires a context with NBCON_PRIO_NORMAL on [CPU X] + * and is scheduled out. + * 4. [Task A] gets running on [CPU X] and sees that the console is + * still owned by a task on [CPU X] with NBON_PRIO_NORMAL. Thus + * [Task A] thinks it is the owner when it is not. */ =20 if (cur->prio !=3D expected_prio) --=20 2.39.2