forked from Minki/linux
b87f02307d
There are reports that the console kthreads block the global console lock when the system is going down, for example, reboot, panic. First part of the solution was to block kthreads in these problematic system states so they stopped handling newly added messages. Second part of the solution is to wait when for the kthreads when they are actively printing. It solves the problem when a message was printed before the system entered the problematic state and the kthreads managed to step in. A busy waiting has to be used because panic() can be called in any context and in an unknown state of the scheduler. There must be a timeout because the kthread might get stuck or sleeping and never release the lock. The timeout 10s is an arbitrary value inspired by the softlockup timeout. Link: https://lore.kernel.org/r/20220610205038.GA3050413@paulmck-ThinkPad-P17-Gen-1 Link: https://lore.kernel.org/r/CAMdYzYpF4FNTBPZsEFeWRuEwSies36QM_As8osPWZSr2q-viEA@mail.gmail.com Signed-off-by: Petr Mladek <pmladek@suse.com> Tested-by: Paul E. McKenney <paulmck@kernel.org> Link: https://lore.kernel.org/r/20220615162805.27962-3-pmladek@suse.com
85 lines
1.9 KiB
C
85 lines
1.9 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/console.h>
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#include <linux/kprobes.h>
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#include <linux/delay.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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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 (this_cpu_read(printk_context) || in_nmi()) {
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int len;
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len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, fmt, args);
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defer_console_output();
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return len;
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}
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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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/**
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* try_block_console_kthreads() - Try to block console kthreads and
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* make the global console_lock() avaialble
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*
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* @timeout_ms: The maximum time (in ms) to wait.
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*
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* Prevent console kthreads from starting processing new messages. Wait
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* until the global console_lock() become available.
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*
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* Context: Can be called in any context.
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*/
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void try_block_console_kthreads(int timeout_ms)
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{
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block_console_kthreads = true;
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/* Do not wait when the console lock could not be safely taken. */
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if (this_cpu_read(printk_context) || in_nmi())
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return;
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while (timeout_ms > 0) {
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if (console_trylock()) {
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console_unlock();
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return;
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}
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udelay(1000);
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timeout_ms -= 1;
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}
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}
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