2 * nmi.c - Safe printk in NMI context
4 * This program is free software; you can redistribute it and/or
5 * modify it under the terms of the GNU General Public License
6 * as published by the Free Software Foundation; either version 2
7 * of the License, or (at your option) any later version.
9 * This program is distributed in the hope that it will be useful,
10 * but WITHOUT ANY WARRANTY; without even the implied warranty of
11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
12 * GNU General Public License for more details.
14 * You should have received a copy of the GNU General Public License
15 * along with this program; if not, see <http://www.gnu.org/licenses/>.
18 #include <linux/preempt.h>
19 #include <linux/spinlock.h>
20 #include <linux/smp.h>
21 #include <linux/cpumask.h>
22 #include <linux/irq_work.h>
23 #include <linux/printk.h>
28 * printk() could not take logbuf_lock in NMI context. Instead,
29 * it uses an alternative implementation that temporary stores
30 * the strings into a per-CPU buffer. The content of the buffer
31 * is later flushed into the main ring buffer via IRQ work.
33 * The alternative implementation is chosen transparently
34 * via @printk_func per-CPU variable.
36 * The implementation allows to flush the strings also from another CPU.
37 * There are situations when we want to make sure that all buffers
38 * were handled or when IRQs are blocked.
40 DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
41 static int printk_nmi_irq_ready;
43 #define NMI_LOG_BUF_LEN (4096 - sizeof(atomic_t) - sizeof(struct irq_work))
46 atomic_t len; /* length of written data */
47 struct irq_work work; /* IRQ work that flushes the buffer */
48 unsigned char buffer[NMI_LOG_BUF_LEN];
50 static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);
53 * Safe printk() for NMI context. It uses a per-CPU buffer to
54 * store the message. NMIs are not nested, so there is always only
55 * one writer running. But the buffer might get flushed from another
56 * CPU, so we need to be careful.
58 static int vprintk_nmi(const char *fmt, va_list args)
60 struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
65 len = atomic_read(&s->len);
67 if (len >= sizeof(s->buffer))
71 * Make sure that all old data have been read before the buffer was
72 * reseted. This is not needed when we just append data.
77 add = vsnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, args);
80 * Do it once again if the buffer has been flushed in the meantime.
81 * Note that atomic_cmpxchg() is an implicit memory barrier that
82 * makes sure that the data were written before updating s->len.
84 if (atomic_cmpxchg(&s->len, len, len + add) != len)
87 /* Get flushed in a more safe context. */
88 if (add && printk_nmi_irq_ready) {
89 /* Make sure that IRQ work is really initialized. */
91 irq_work_queue(&s->work);
98 * printk one line from the temporary buffer from @start index until
99 * and including the @end index.
101 static void print_nmi_seq_line(struct nmi_seq_buf *s, int start, int end)
103 const char *buf = s->buffer + start;
105 printk("%.*s", (end - start) + 1, buf);
109 * Flush data from the associated per_CPU buffer. The function
110 * can be called either via IRQ work or independently.
112 static void __printk_nmi_flush(struct irq_work *work)
114 static raw_spinlock_t read_lock =
115 __RAW_SPIN_LOCK_INITIALIZER(read_lock);
116 struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work);
122 * The lock has two functions. First, one reader has to flush all
123 * available message to make the lockless synchronization with
124 * writers easier. Second, we do not want to mix messages from
125 * different CPUs. This is especially important when printing
128 raw_spin_lock_irqsave(&read_lock, flags);
132 len = atomic_read(&s->len);
135 * This is just a paranoid check that nobody has manipulated
136 * the buffer an unexpected way. If we printed something then
137 * @len must only increase.
140 pr_err("printk_nmi_flush: internal error: i=%d >= len=%zu\n",
144 goto out; /* Someone else has already flushed the buffer. */
146 /* Make sure that data has been written up to the @len */
149 size = min(len, sizeof(s->buffer));
152 /* Print line by line. */
153 for (; i < size; i++) {
154 if (s->buffer[i] == '\n') {
155 print_nmi_seq_line(s, last_i, i);
159 /* Check if there was a partial line. */
161 print_nmi_seq_line(s, last_i, size - 1);
166 * Check that nothing has got added in the meantime and truncate
167 * the buffer. Note that atomic_cmpxchg() is an implicit memory
168 * barrier that makes sure that the data were copied before
171 if (atomic_cmpxchg(&s->len, len, 0) != len)
175 raw_spin_unlock_irqrestore(&read_lock, flags);
179 * printk_nmi_flush - flush all per-cpu nmi buffers.
181 * The buffers are flushed automatically via IRQ work. This function
182 * is useful only when someone wants to be sure that all buffers have
183 * been flushed at some point.
185 void printk_nmi_flush(void)
189 for_each_possible_cpu(cpu)
190 __printk_nmi_flush(&per_cpu(nmi_print_seq, cpu).work);
193 void __init printk_nmi_init(void)
197 for_each_possible_cpu(cpu) {
198 struct nmi_seq_buf *s = &per_cpu(nmi_print_seq, cpu);
200 init_irq_work(&s->work, __printk_nmi_flush);
203 /* Make sure that IRQ works are initialized before enabling. */
205 printk_nmi_irq_ready = 1;
207 /* Flush pending messages that did not have scheduled IRQ works. */
211 void printk_nmi_enter(void)
213 this_cpu_write(printk_func, vprintk_nmi);
216 void printk_nmi_exit(void)
218 this_cpu_write(printk_func, vprintk_default);