kernel/printk/nmi.c

   1 /*
   2  * nmi.c - Safe printk in NMI context
   3  *
   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.
   8  *
   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.
  13  *
  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/>.
  16  */
  17
  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>
  24
  25 #include "internal.h"
  26
  27 /*
  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.
  32  *
  33  * The alternative implementation is chosen transparently
  34  * via @printk_func per-CPU variable.
  35  *
  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.
  39  */
  40 DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
  41 static int printk_nmi_irq_ready;
  42 atomic_t nmi_message_lost;
  43
  44 #define NMI_LOG_BUF_LEN ((1 << CONFIG_NMI_LOG_BUF_SHIFT) -              \
  45                          sizeof(atomic_t) - sizeof(struct irq_work))
  46
  47 struct nmi_seq_buf {
  48         atomic_t                len;    /* length of written data */
  49         struct irq_work         work;   /* IRQ work that flushes the buffer */
  50         unsigned char           buffer[NMI_LOG_BUF_LEN];
  51 };
  52 static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);
  53
  54 /*
  55  * Safe printk() for NMI context. It uses a per-CPU buffer to
  56  * store the message. NMIs are not nested, so there is always only
  57  * one writer running. But the buffer might get flushed from another
  58  * CPU, so we need to be careful.
  59  */
  60 static int vprintk_nmi(const char *fmt, va_list args)
  61 {
  62         struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
  63         int add = 0;
  64         size_t len;
  65
  66 again:
  67         len = atomic_read(&s->len);
  68
  69         if (len >= sizeof(s->buffer)) {
  70                 atomic_inc(&nmi_message_lost);
  71                 return 0;
  72         }
  73
  74         /*
  75          * Make sure that all old data have been read before the buffer was
  76          * reseted. This is not needed when we just append data.
  77          */
  78         if (!len)
  79                 smp_rmb();
  80
  81         add = vsnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, args);
  82
  83         /*
  84          * Do it once again if the buffer has been flushed in the meantime.
  85          * Note that atomic_cmpxchg() is an implicit memory barrier that
  86          * makes sure that the data were written before updating s->len.
  87          */
  88         if (atomic_cmpxchg(&s->len, len, len + add) != len)
  89                 goto again;
  90
  91         /* Get flushed in a more safe context. */
  92         if (add && printk_nmi_irq_ready) {
  93                 /* Make sure that IRQ work is really initialized. */
  94                 smp_rmb();
  95                 irq_work_queue(&s->work);
  96         }
  97
  98         return add;
  99 }
 100
 101 /*
 102  * printk one line from the temporary buffer from @start index until
 103  * and including the @end index.
 104  */
 105 static void print_nmi_seq_line(struct nmi_seq_buf *s, int start, int end)
 106 {
 107         const char *buf = s->buffer + start;
 108
 109         printk("%.*s", (end - start) + 1, buf);
 110 }
 111
 112 /*
 113  * Flush data from the associated per_CPU buffer. The function
 114  * can be called either via IRQ work or independently.
 115  */
 116 static void __printk_nmi_flush(struct irq_work *work)
 117 {
 118         static raw_spinlock_t read_lock =
 119                 __RAW_SPIN_LOCK_INITIALIZER(read_lock);
 120         struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work);
 121         unsigned long flags;
 122         size_t len, size;
 123         int i, last_i;
 124
 125         /*
 126          * The lock has two functions. First, one reader has to flush all
 127          * available message to make the lockless synchronization with
 128          * writers easier. Second, we do not want to mix messages from
 129          * different CPUs. This is especially important when printing
 130          * a backtrace.
 131          */
 132         raw_spin_lock_irqsave(&read_lock, flags);
 133
 134         i = 0;
 135 more:
 136         len = atomic_read(&s->len);
 137
 138         /*
 139          * This is just a paranoid check that nobody has manipulated
 140          * the buffer an unexpected way. If we printed something then
 141          * @len must only increase.
 142          */
 143         if (i && i >= len)
 144                 pr_err("printk_nmi_flush: internal error: i=%d >= len=%zu\n",
 145                        i, len);
 146
 147         if (!len)
 148                 goto out; /* Someone else has already flushed the buffer. */
 149
 150         /* Make sure that data has been written up to the @len */
 151         smp_rmb();
 152
 153         size = min(len, sizeof(s->buffer));
 154         last_i = i;
 155
 156         /* Print line by line. */
 157         for (; i < size; i++) {
 158                 if (s->buffer[i] == '\n') {
 159                         print_nmi_seq_line(s, last_i, i);
 160                         last_i = i + 1;
 161                 }
 162         }
 163         /* Check if there was a partial line. */
 164         if (last_i < size) {
 165                 print_nmi_seq_line(s, last_i, size - 1);
 166                 pr_cont("\n");
 167         }
 168
 169         /*
 170          * Check that nothing has got added in the meantime and truncate
 171          * the buffer. Note that atomic_cmpxchg() is an implicit memory
 172          * barrier that makes sure that the data were copied before
 173          * updating s->len.
 174          */
 175         if (atomic_cmpxchg(&s->len, len, 0) != len)
 176                 goto more;
 177
 178 out:
 179         raw_spin_unlock_irqrestore(&read_lock, flags);
 180 }
 181
 182 /**
 183  * printk_nmi_flush - flush all per-cpu nmi buffers.
 184  *
 185  * The buffers are flushed automatically via IRQ work. This function
 186  * is useful only when someone wants to be sure that all buffers have
 187  * been flushed at some point.
 188  */
 189 void printk_nmi_flush(void)
 190 {
 191         int cpu;
 192
 193         for_each_possible_cpu(cpu)
 194                 __printk_nmi_flush(&per_cpu(nmi_print_seq, cpu).work);
 195 }
 196
 197 void __init printk_nmi_init(void)
 198 {
 199         int cpu;
 200
 201         for_each_possible_cpu(cpu) {
 202                 struct nmi_seq_buf *s = &per_cpu(nmi_print_seq, cpu);
 203
 204                 init_irq_work(&s->work, __printk_nmi_flush);
 205         }
 206
 207         /* Make sure that IRQ works are initialized before enabling. */
 208         smp_wmb();
 209         printk_nmi_irq_ready = 1;
 210
 211         /* Flush pending messages that did not have scheduled IRQ works. */
 212         printk_nmi_flush();
 213 }
 214
 215 void printk_nmi_enter(void)
 216 {
 217         this_cpu_write(printk_func, vprintk_nmi);
 218 }
 219
 220 void printk_nmi_exit(void)
 221 {
 222         this_cpu_write(printk_func, vprintk_default);
 223 }