Konstantin Demin
c3d09a3e94
imported from https://salsa.debian.org/kernel-team/linux.git commit 9d5cc9d9d6501d7f1dd7e194d4b245bd0b6c6a22 version 6.11.4-1
111 lines
4.7 KiB
Diff
111 lines
4.7 KiB
Diff
From: John Ogness <john.ogness@linutronix.de>
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Date: Tue, 20 Aug 2024 08:35:31 +0206
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Subject: [PATCH 05/54] printk: nbcon: Clarify rules of the owner/waiter
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matching
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Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.11/older/patches-6.11-rt7.tar.xz
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The functions nbcon_owner_matches() and nbcon_waiter_matches()
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use a minimal set of data to determine if a context matches.
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The existing kerneldoc and comments were not clear enough and
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caused the printk folks to re-prove that the functions are
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indeed reliable in all cases.
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Update and expand the explanations so that it is clear that the
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implementations are sufficient for all cases.
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Signed-off-by: John Ogness <john.ogness@linutronix.de>
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Reviewed-by: Petr Mladek <pmladek@suse.com>
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Link: https://lore.kernel.org/r/20240820063001.36405-6-john.ogness@linutronix.de
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Signed-off-by: Petr Mladek <pmladek@suse.com>
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Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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---
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kernel/printk/nbcon.c | 56 +++++++++++++++++++++++++++++++++++++++++---------
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1 file changed, 46 insertions(+), 10 deletions(-)
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--- a/kernel/printk/nbcon.c
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+++ b/kernel/printk/nbcon.c
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@@ -228,6 +228,13 @@ static int nbcon_context_try_acquire_dir
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struct nbcon_state new;
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do {
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+ /*
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+ * Panic does not imply that the console is owned. However, it
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+ * is critical that non-panic CPUs during panic are unable to
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+ * acquire ownership in order to satisfy the assumptions of
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+ * nbcon_waiter_matches(). In particular, the assumption that
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+ * lower priorities are ignored during panic.
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+ */
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if (other_cpu_in_panic())
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return -EPERM;
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@@ -259,18 +266,29 @@ static bool nbcon_waiter_matches(struct
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/*
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* The request context is well defined by the @req_prio because:
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*
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- * - Only a context with a higher priority can take over the request.
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+ * - Only a context with a priority higher than the owner can become
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+ * a waiter.
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+ * - Only a context with a priority higher than the waiter can
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+ * directly take over the request.
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* - There are only three priorities.
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* - Only one CPU is allowed to request PANIC priority.
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* - Lower priorities are ignored during panic() until reboot.
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*
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* As a result, the following scenario is *not* possible:
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*
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- * 1. Another context with a higher priority directly takes ownership.
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- * 2. The higher priority context releases the ownership.
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- * 3. A lower priority context takes the ownership.
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- * 4. Another context with the same priority as this context
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+ * 1. This context is currently a waiter.
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+ * 2. Another context with a higher priority than this context
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+ * directly takes ownership.
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+ * 3. The higher priority context releases the ownership.
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+ * 4. Another lower priority context takes the ownership.
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+ * 5. Another context with the same priority as this context
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* creates a request and starts waiting.
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+ *
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+ * Event #1 implies this context is EMERGENCY.
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+ * Event #2 implies the new context is PANIC.
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+ * Event #3 occurs when panic() has flushed the console.
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+ * Events #4 and #5 are not possible due to the other_cpu_in_panic()
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+ * check in nbcon_context_try_acquire_direct().
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*/
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return (cur->req_prio == expected_prio);
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@@ -578,11 +596,29 @@ static bool nbcon_owner_matches(struct n
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int expected_prio)
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{
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/*
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- * Since consoles can only be acquired by higher priorities,
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- * owning contexts are uniquely identified by @prio. However,
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- * since contexts can unexpectedly lose ownership, it is
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- * possible that later another owner appears with the same
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- * priority. For this reason @cpu is also needed.
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+ * A similar function, nbcon_waiter_matches(), only deals with
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+ * EMERGENCY and PANIC priorities. However, this function must also
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+ * deal with the NORMAL priority, which requires additional checks
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+ * and constraints.
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+ *
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+ * For the case where preemption and interrupts are disabled, it is
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+ * enough to also verify that the owning CPU has not changed.
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+ *
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+ * For the case where preemption or interrupts are enabled, an
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+ * external synchronization method *must* be used. In particular,
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+ * the driver-specific locking mechanism used in device_lock()
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+ * (including disabling migration) should be used. It prevents
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+ * scenarios such as:
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+ *
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+ * 1. [Task A] owns a context with NBCON_PRIO_NORMAL on [CPU X] and
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+ * is scheduled out.
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+ * 2. Another context takes over the lock with NBCON_PRIO_EMERGENCY
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+ * and releases it.
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+ * 3. [Task B] acquires a context with NBCON_PRIO_NORMAL on [CPU X]
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+ * and is scheduled out.
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+ * 4. [Task A] gets running on [CPU X] and sees that the console is
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+ * still owned by a task on [CPU X] with NBON_PRIO_NORMAL. Thus
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+ * [Task A] thinks it is the owner when it is not.
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*/
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if (cur->prio != expected_prio)
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