272 lines
7.1 KiB
C
272 lines
7.1 KiB
C
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/*
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* nmi.c - Safe printk in NMI context
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; either version 2
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* of the License, or (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, see <http://www.gnu.org/licenses/>.
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*/
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#include <linux/preempt.h>
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#include <linux/spinlock.h>
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#include <linux/debug_locks.h>
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#include <linux/smp.h>
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#include <linux/cpumask.h>
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#include <linux/irq_work.h>
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#include <linux/printk.h>
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#include "internal.h"
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/*
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* printk() could not take logbuf_lock in NMI context. Instead,
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* it uses an alternative implementation that temporary stores
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* the strings into a per-CPU buffer. The content of the buffer
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* is later flushed into the main ring buffer via IRQ work.
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*
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* The alternative implementation is chosen transparently
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* via @printk_func per-CPU variable.
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*
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* The implementation allows to flush the strings also from another CPU.
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* There are situations when we want to make sure that all buffers
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* were handled or when IRQs are blocked.
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*/
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DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
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static int printk_nmi_irq_ready;
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atomic_t nmi_message_lost;
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#define NMI_LOG_BUF_LEN ((1 << CONFIG_NMI_LOG_BUF_SHIFT) - \
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sizeof(atomic_t) - sizeof(struct irq_work))
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struct nmi_seq_buf {
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atomic_t len; /* length of written data */
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struct irq_work work; /* IRQ work that flushes the buffer */
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unsigned char buffer[NMI_LOG_BUF_LEN];
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};
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static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);
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/*
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* Safe printk() for NMI context. It uses a per-CPU buffer to
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* store the message. NMIs are not nested, so there is always only
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* one writer running. But the buffer might get flushed from another
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* CPU, so we need to be careful.
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*/
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static int vprintk_nmi(const char *fmt, va_list args)
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{
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struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
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int add = 0;
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size_t len;
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va_list ap;
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again:
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len = atomic_read(&s->len);
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if (len >= sizeof(s->buffer)) {
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atomic_inc(&nmi_message_lost);
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return 0;
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}
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/*
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* Make sure that all old data have been read before the buffer was
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* reseted. This is not needed when we just append data.
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*/
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if (!len)
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smp_rmb();
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va_copy(ap, args);
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add = vsnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, ap);
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va_end(ap);
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/*
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* Do it once again if the buffer has been flushed in the meantime.
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* Note that atomic_cmpxchg() is an implicit memory barrier that
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* makes sure that the data were written before updating s->len.
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*/
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if (atomic_cmpxchg(&s->len, len, len + add) != len)
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goto again;
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/* Get flushed in a more safe context. */
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if (add && printk_nmi_irq_ready) {
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/* Make sure that IRQ work is really initialized. */
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smp_rmb();
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irq_work_queue(&s->work);
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}
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return add;
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}
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static void printk_nmi_flush_line(const char *text, int len)
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{
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/*
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* The buffers are flushed in NMI only on panic. The messages must
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* go only into the ring buffer at this stage. Consoles will get
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* explicitly called later when a crashdump is not generated.
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*/
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if (in_nmi())
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printk_deferred("%.*s", len, text);
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else
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printk("%.*s", len, text);
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}
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/*
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* printk one line from the temporary buffer from @start index until
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* and including the @end index.
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*/
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static void printk_nmi_flush_seq_line(struct nmi_seq_buf *s,
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int start, int end)
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{
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const char *buf = s->buffer + start;
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printk_nmi_flush_line(buf, (end - start) + 1);
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}
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/*
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* Flush data from the associated per_CPU buffer. The function
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* can be called either via IRQ work or independently.
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*/
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static void __printk_nmi_flush(struct irq_work *work)
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{
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static raw_spinlock_t read_lock =
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__RAW_SPIN_LOCK_INITIALIZER(read_lock);
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struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work);
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unsigned long flags;
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size_t len, size;
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int i, last_i;
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/*
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* The lock has two functions. First, one reader has to flush all
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* available message to make the lockless synchronization with
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* writers easier. Second, we do not want to mix messages from
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* different CPUs. This is especially important when printing
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* a backtrace.
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*/
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raw_spin_lock_irqsave(&read_lock, flags);
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i = 0;
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more:
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len = atomic_read(&s->len);
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/*
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* This is just a paranoid check that nobody has manipulated
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* the buffer an unexpected way. If we printed something then
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* @len must only increase.
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*/
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if (i && i >= len) {
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const char *msg = "printk_nmi_flush: internal error\n";
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printk_nmi_flush_line(msg, strlen(msg));
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}
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if (!len)
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goto out; /* Someone else has already flushed the buffer. */
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/* Make sure that data has been written up to the @len */
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smp_rmb();
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size = min(len, sizeof(s->buffer));
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last_i = i;
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/* Print line by line. */
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for (; i < size; i++) {
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if (s->buffer[i] == '\n') {
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printk_nmi_flush_seq_line(s, last_i, i);
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last_i = i + 1;
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}
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}
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/* Check if there was a partial line. */
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if (last_i < size) {
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printk_nmi_flush_seq_line(s, last_i, size - 1);
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printk_nmi_flush_line("\n", strlen("\n"));
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}
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/*
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* Check that nothing has got added in the meantime and truncate
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* the buffer. Note that atomic_cmpxchg() is an implicit memory
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* barrier that makes sure that the data were copied before
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* updating s->len.
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*/
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if (atomic_cmpxchg(&s->len, len, 0) != len)
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goto more;
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out:
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raw_spin_unlock_irqrestore(&read_lock, flags);
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}
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/**
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* printk_nmi_flush - flush all per-cpu nmi buffers.
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*
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* The buffers are flushed automatically via IRQ work. This function
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* is useful only when someone wants to be sure that all buffers have
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* been flushed at some point.
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*/
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void printk_nmi_flush(void)
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{
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int cpu;
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for_each_possible_cpu(cpu)
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__printk_nmi_flush(&per_cpu(nmi_print_seq, cpu).work);
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}
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/**
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* printk_nmi_flush_on_panic - flush all per-cpu nmi buffers when the system
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* goes down.
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*
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* Similar to printk_nmi_flush() but it can be called even in NMI context when
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* the system goes down. It does the best effort to get NMI messages into
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* the main ring buffer.
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*
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* Note that it could try harder when there is only one CPU online.
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*/
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void printk_nmi_flush_on_panic(void)
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{
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/*
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* Make sure that we could access the main ring buffer.
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* Do not risk a double release when more CPUs are up.
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*/
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if (in_nmi() && raw_spin_is_locked(&logbuf_lock)) {
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if (num_online_cpus() > 1)
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return;
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debug_locks_off();
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raw_spin_lock_init(&logbuf_lock);
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}
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printk_nmi_flush();
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}
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void __init printk_nmi_init(void)
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{
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int cpu;
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for_each_possible_cpu(cpu) {
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struct nmi_seq_buf *s = &per_cpu(nmi_print_seq, cpu);
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init_irq_work(&s->work, __printk_nmi_flush);
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}
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/* Make sure that IRQ works are initialized before enabling. */
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smp_wmb();
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printk_nmi_irq_ready = 1;
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/* Flush pending messages that did not have scheduled IRQ works. */
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printk_nmi_flush();
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}
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void printk_nmi_enter(void)
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{
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this_cpu_write(printk_func, vprintk_nmi);
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}
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void printk_nmi_exit(void)
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{
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this_cpu_write(printk_func, vprintk_default);
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}
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