blob: b69eb8a2876fc2ec7ba1af9e5cbc000737cd47fd [file] [log] [blame]
/*
* nmi.c - Safe printk in NMI context
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version 2
* of the License, or (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, see <http://www.gnu.org/licenses/>.
*/
#include <linux/preempt.h>
#include <linux/spinlock.h>
#include <linux/debug_locks.h>
#include <linux/smp.h>
#include <linux/cpumask.h>
#include <linux/irq_work.h>
#include <linux/printk.h>
#include "internal.h"
/*
* printk() could not take logbuf_lock in NMI context. Instead,
* it uses an alternative implementation that temporary stores
* the strings into a per-CPU buffer. The content of the buffer
* is later flushed into the main ring buffer via IRQ work.
*
* The alternative implementation is chosen transparently
* via @printk_func per-CPU variable.
*
* The implementation allows to flush the strings also from another CPU.
* There are situations when we want to make sure that all buffers
* were handled or when IRQs are blocked.
*/
DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
static int printk_nmi_irq_ready;
atomic_t nmi_message_lost;
#define NMI_LOG_BUF_LEN ((1 << CONFIG_NMI_LOG_BUF_SHIFT) - \
sizeof(atomic_t) - sizeof(struct irq_work))
struct nmi_seq_buf {
atomic_t len; /* length of written data */
struct irq_work work; /* IRQ work that flushes the buffer */
unsigned char buffer[NMI_LOG_BUF_LEN];
};
static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);
/*
* Safe printk() for NMI context. It uses a per-CPU buffer to
* store the message. NMIs are not nested, so there is always only
* one writer running. But the buffer might get flushed from another
* CPU, so we need to be careful.
*/
static int vprintk_nmi(const char *fmt, va_list args)
{
struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
int add = 0;
size_t len;
again:
len = atomic_read(&s->len);
if (len >= sizeof(s->buffer)) {
atomic_inc(&nmi_message_lost);
return 0;
}
/*
* Make sure that all old data have been read before the buffer was
* reseted. This is not needed when we just append data.
*/
if (!len)
smp_rmb();
add = vsnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, args);
/*
* Do it once again if the buffer has been flushed in the meantime.
* Note that atomic_cmpxchg() is an implicit memory barrier that
* makes sure that the data were written before updating s->len.
*/
if (atomic_cmpxchg(&s->len, len, len + add) != len)
goto again;
/* Get flushed in a more safe context. */
if (add && printk_nmi_irq_ready) {
/* Make sure that IRQ work is really initialized. */
smp_rmb();
irq_work_queue(&s->work);
}
return add;
}
/*
* printk one line from the temporary buffer from @start index until
* and including the @end index.
*/
static void print_nmi_seq_line(struct nmi_seq_buf *s, int start, int end)
{
const char *buf = s->buffer + start;
/*
* The buffers are flushed in NMI only on panic. The messages must
* go only into the ring buffer at this stage. Consoles will get
* explicitly called later when a crashdump is not generated.
*/
if (in_nmi())
printk_deferred("%.*s", (end - start) + 1, buf);
else
printk("%.*s", (end - start) + 1, buf);
}
/*
* Flush data from the associated per_CPU buffer. The function
* can be called either via IRQ work or independently.
*/
static void __printk_nmi_flush(struct irq_work *work)
{
static raw_spinlock_t read_lock =
__RAW_SPIN_LOCK_INITIALIZER(read_lock);
struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work);
unsigned long flags;
size_t len, size;
int i, last_i;
/*
* The lock has two functions. First, one reader has to flush all
* available message to make the lockless synchronization with
* writers easier. Second, we do not want to mix messages from
* different CPUs. This is especially important when printing
* a backtrace.
*/
raw_spin_lock_irqsave(&read_lock, flags);
i = 0;
more:
len = atomic_read(&s->len);
/*
* This is just a paranoid check that nobody has manipulated
* the buffer an unexpected way. If we printed something then
* @len must only increase.
*/
if (i && i >= len)
pr_err("printk_nmi_flush: internal error: i=%d >= len=%zu\n",
i, len);
if (!len)
goto out; /* Someone else has already flushed the buffer. */
/* Make sure that data has been written up to the @len */
smp_rmb();
size = min(len, sizeof(s->buffer));
last_i = i;
/* Print line by line. */
for (; i < size; i++) {
if (s->buffer[i] == '\n') {
print_nmi_seq_line(s, last_i, i);
last_i = i + 1;
}
}
/* Check if there was a partial line. */
if (last_i < size) {
print_nmi_seq_line(s, last_i, size - 1);
pr_cont("\n");
}
/*
* Check that nothing has got added in the meantime and truncate
* the buffer. Note that atomic_cmpxchg() is an implicit memory
* barrier that makes sure that the data were copied before
* updating s->len.
*/
if (atomic_cmpxchg(&s->len, len, 0) != len)
goto more;
out:
raw_spin_unlock_irqrestore(&read_lock, flags);
}
/**
* printk_nmi_flush - flush all per-cpu nmi buffers.
*
* The buffers are flushed automatically via IRQ work. This function
* is useful only when someone wants to be sure that all buffers have
* been flushed at some point.
*/
void printk_nmi_flush(void)
{
int cpu;
for_each_possible_cpu(cpu)
__printk_nmi_flush(&per_cpu(nmi_print_seq, cpu).work);
}
/**
* printk_nmi_flush_on_panic - flush all per-cpu nmi buffers when the system
* goes down.
*
* Similar to printk_nmi_flush() but it can be called even in NMI context when
* the system goes down. It does the best effort to get NMI messages into
* the main ring buffer.
*
* Note that it could try harder when there is only one CPU online.
*/
void printk_nmi_flush_on_panic(void)
{
/*
* Make sure that we could access the main ring buffer.
* Do not risk a double release when more CPUs are up.
*/
if (in_nmi() && raw_spin_is_locked(&logbuf_lock)) {
if (num_online_cpus() > 1)
return;
debug_locks_off();
raw_spin_lock_init(&logbuf_lock);
}
printk_nmi_flush();
}
void __init printk_nmi_init(void)
{
int cpu;
for_each_possible_cpu(cpu) {
struct nmi_seq_buf *s = &per_cpu(nmi_print_seq, cpu);
init_irq_work(&s->work, __printk_nmi_flush);
}
/* Make sure that IRQ works are initialized before enabling. */
smp_wmb();
printk_nmi_irq_ready = 1;
/* Flush pending messages that did not have scheduled IRQ works. */
printk_nmi_flush();
}
void printk_nmi_enter(void)
{
this_cpu_write(printk_func, vprintk_nmi);
}
void printk_nmi_exit(void)
{
this_cpu_write(printk_func, vprintk_default);
}