John Ogness
b727c2c9c3
[ Upstream commit 0161e2d6950fe66cf6ac1c10d945bae971f33667 ]
The documentation of printk_cpu_sync_get() clearly states
that the owner must never perform any activities where it waits
for a CPU. For legacy printing there can be spinning on the
console_lock and on the port lock. Therefore legacy printing
must be deferred when holding the printk_cpu_sync.
Note that in the case of emergency states, atomic consoles
are not prevented from printing when printk is deferred. This
is appropriate because they do not spin-wait indefinitely for
other CPUs.
Reported-by: Rik van Riel <riel@surriel.com>
Closes: https://lore.kernel.org/r/20240715232052.73eb7fb1@imladris.surriel.com
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Fixes: 55d6af1d66 ("lib/nmi_backtrace: explicitly serialize banner and regs")
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20241209111746.192559-3-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
76 lines
1.6 KiB
C
76 lines
1.6 KiB
C
// SPDX-License-Identifier: GPL-2.0-or-later
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/*
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* printk_safe.c - Safe printk for printk-deadlock-prone contexts
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*/
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#include <linux/preempt.h>
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#include <linux/kdb.h>
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#include <linux/smp.h>
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#include <linux/cpumask.h>
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#include <linux/printk.h>
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#include <linux/kprobes.h>
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#include "internal.h"
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static DEFINE_PER_CPU(int, printk_context);
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/* Can be preempted by NMI. */
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void __printk_safe_enter(void)
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{
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this_cpu_inc(printk_context);
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}
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/* Can be preempted by NMI. */
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void __printk_safe_exit(void)
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{
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this_cpu_dec(printk_context);
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}
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void __printk_deferred_enter(void)
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{
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cant_migrate();
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__printk_safe_enter();
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}
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void __printk_deferred_exit(void)
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{
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cant_migrate();
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__printk_safe_exit();
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}
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bool is_printk_legacy_deferred(void)
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{
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/*
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* The per-CPU variable @printk_context can be read safely in any
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* context. CPU migration is always disabled when set.
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*
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* A context holding the printk_cpu_sync must not spin waiting for
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* another CPU. For legacy printing, it could be the console_lock
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* or the port lock.
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*/
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return (force_legacy_kthread() ||
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this_cpu_read(printk_context) ||
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in_nmi() ||
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is_printk_cpu_sync_owner());
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}
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asmlinkage int vprintk(const char *fmt, va_list args)
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{
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#ifdef CONFIG_KGDB_KDB
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/* Allow to pass printk() to kdb but avoid a recursion. */
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if (unlikely(kdb_trap_printk && kdb_printf_cpu < 0))
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return vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args);
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#endif
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/*
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* Use the main logbuf even in NMI. But avoid calling console
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* drivers that might have their own locks.
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*/
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if (is_printk_legacy_deferred())
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return vprintk_deferred(fmt, args);
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/* No obstacles. */
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return vprintk_default(fmt, args);
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}
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EXPORT_SYMBOL(vprintk);
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