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/debug_locks.h>
  21 #include <linux/smp.h>
  22 #include <linux/cpumask.h>
  23 #include <linux/irq_work.h>
  24 #include <linux/printk.h>
  25
  26 #include "internal.h"
  27
  28 /*
  29  * printk() could not take logbuf_lock in NMI context. Instead,
  30  * it uses an alternative implementation that temporary stores
  31  * the strings into a per-CPU buffer. The content of the buffer
  32  * is later flushed into the main ring buffer via IRQ work.
  33  *
  34  * The alternative implementation is chosen transparently
  35  * via @printk_func per-CPU variable.
  36  *
  37  * The implementation allows to flush the strings also from another CPU.
  38  * There are situations when we want to make sure that all buffers
  39  * were handled or when IRQs are blocked.
  40  */
  41 DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
  42 static int printk_nmi_irq_ready;
  43 atomic_t nmi_message_lost;
  44
  45 #define NMI_LOG_BUF_LEN ((1 << CONFIG_NMI_LOG_BUF_SHIFT) -              \
  46                          sizeof(atomic_t) - sizeof(struct irq_work))
  47
  48 struct nmi_seq_buf {
  49         atomic_t                len;    /* length of written data */
  50         struct irq_work         work;   /* IRQ work that flushes the buffer */
  51         unsigned char           buffer[NMI_LOG_BUF_LEN];
  52 };
  53 static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);
  54
  55 /*
  56  * Safe printk() for NMI context. It uses a per-CPU buffer to
  57  * store the message. NMIs are not nested, so there is always only
  58  * one writer running. But the buffer might get flushed from another
  59  * CPU, so we need to be careful.
  60  */
  61 static int vprintk_nmi(const char *fmt, va_list args)
  62 {
  63         struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
  64         int add = 0;
  65         size_t len;
  66
  67 again:
  68         len = atomic_read(&s->len);
  69
  70         /* The trailing '\0' is not counted into len. */
  71         if (len >= sizeof(s->buffer) - 1) {
  72                 atomic_inc(&nmi_message_lost);
  73                 return 0;
  74         }
  75
  76         /*
  77          * Make sure that all old data have been read before the buffer was
  78          * reseted. This is not needed when we just append data.
  79          */
  80         if (!len)
  81                 smp_rmb();
  82
  83         add = vscnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, args);
  84
  85         /*
  86          * Do it once again if the buffer has been flushed in the meantime.
  87          * Note that atomic_cmpxchg() is an implicit memory barrier that
  88          * makes sure that the data were written before updating s->len.
  89          */
  90         if (atomic_cmpxchg(&s->len, len, len + add) != len)
  91                 goto again;
  92
  93         /* Get flushed in a more safe context. */
  94         if (add && printk_nmi_irq_ready) {
  95                 /* Make sure that IRQ work is really initialized. */
  96                 smp_rmb();
  97                 irq_work_queue(&s->work);
  98         }
  99
 100         return add;
 101 }
 102
 103 static void printk_nmi_flush_line(const char *text, int len)
 104 {
 105         /*
 106          * The buffers are flushed in NMI only on panic.  The messages must
 107          * go only into the ring buffer at this stage.  Consoles will get
 108          * explicitly called later when a crashdump is not generated.
 109          */
 110         if (in_nmi())
 111                 printk_deferred("%.*s", len, text);
 112         else
 113                 printk("%.*s", len, text);
 114
 115 }
 116
 117 /* printk part of the temporary buffer line by line */
 118 static int printk_nmi_flush_buffer(const char *start, size_t len)
 119 {
 120         const char *c, *end;
 121         bool header;
 122
 123         c = start;
 124         end = start + len;
 125         header = true;
 126
 127         /* Print line by line. */
 128         while (c < end) {
 129                 if (*c == '\n') {
 130                         printk_nmi_flush_line(start, c - start + 1);
 131                         start = ++c;
 132                         header = true;
 133                         continue;
 134                 }
 135
 136                 /* Handle continuous lines or missing new line. */
 137                 if ((c + 1 < end) && printk_get_level(c)) {
 138                         if (header) {
 139                                 c = printk_skip_level(c);
 140                                 continue;
 141                         }
 142
 143                         printk_nmi_flush_line(start, c - start);
 144                         start = c++;
 145                         header = true;
 146                         continue;
 147                 }
 148
 149                 header = false;
 150                 c++;
 151         }
 152
 153         /* Check if there was a partial line. Ignore pure header. */
 154         if (start < end && !header) {
 155                 static const char newline[] = KERN_CONT "\n";
 156
 157                 printk_nmi_flush_line(start, end - start);
 158                 printk_nmi_flush_line(newline, strlen(newline));
 159         }
 160
 161         return len;
 162 }
 163
 164 /*
 165  * Flush data from the associated per_CPU buffer. The function
 166  * can be called either via IRQ work or independently.
 167  */
 168 static void __printk_nmi_flush(struct irq_work *work)
 169 {
 170         static raw_spinlock_t read_lock =
 171                 __RAW_SPIN_LOCK_INITIALIZER(read_lock);
 172         struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work);
 173         unsigned long flags;
 174         size_t len;
 175         int i;
 176
 177         /*
 178          * The lock has two functions. First, one reader has to flush all
 179          * available message to make the lockless synchronization with
 180          * writers easier. Second, we do not want to mix messages from
 181          * different CPUs. This is especially important when printing
 182          * a backtrace.
 183          */
 184         raw_spin_lock_irqsave(&read_lock, flags);
 185
 186         i = 0;
 187 more:
 188         len = atomic_read(&s->len);
 189
 190         /*
 191          * This is just a paranoid check that nobody has manipulated
 192          * the buffer an unexpected way. If we printed something then
 193          * @len must only increase. Also it should never overflow the
 194          * buffer size.
 195          */
 196         if ((i && i >= len) || len > sizeof(s->buffer)) {
 197                 const char *msg = "printk_nmi_flush: internal error\n";
 198
 199                 printk_nmi_flush_line(msg, strlen(msg));
 200                 len = 0;
 201         }
 202
 203         if (!len)
 204                 goto out; /* Someone else has already flushed the buffer. */
 205
 206         /* Make sure that data has been written up to the @len */
 207         smp_rmb();
 208         i += printk_nmi_flush_buffer(s->buffer + i, len - i);
 209
 210         /*
 211          * Check that nothing has got added in the meantime and truncate
 212          * the buffer. Note that atomic_cmpxchg() is an implicit memory
 213          * barrier that makes sure that the data were copied before
 214          * updating s->len.
 215          */
 216         if (atomic_cmpxchg(&s->len, len, 0) != len)
 217                 goto more;
 218
 219 out:
 220         raw_spin_unlock_irqrestore(&read_lock, flags);
 221 }
 222
 223 /**
 224  * printk_nmi_flush - flush all per-cpu nmi buffers.
 225  *
 226  * The buffers are flushed automatically via IRQ work. This function
 227  * is useful only when someone wants to be sure that all buffers have
 228  * been flushed at some point.
 229  */
 230 void printk_nmi_flush(void)
 231 {
 232         int cpu;
 233
 234         for_each_possible_cpu(cpu)
 235                 __printk_nmi_flush(&per_cpu(nmi_print_seq, cpu).work);
 236 }
 237
 238 /**
 239  * printk_nmi_flush_on_panic - flush all per-cpu nmi buffers when the system
 240  *      goes down.
 241  *
 242  * Similar to printk_nmi_flush() but it can be called even in NMI context when
 243  * the system goes down. It does the best effort to get NMI messages into
 244  * the main ring buffer.
 245  *
 246  * Note that it could try harder when there is only one CPU online.
 247  */
 248 void printk_nmi_flush_on_panic(void)
 249 {
 250         /*
 251          * Make sure that we could access the main ring buffer.
 252          * Do not risk a double release when more CPUs are up.
 253          */
 254         if (in_nmi() && raw_spin_is_locked(&logbuf_lock)) {
 255                 if (num_online_cpus() > 1)
 256                         return;
 257
 258                 debug_locks_off();
 259                 raw_spin_lock_init(&logbuf_lock);
 260         }
 261
 262         printk_nmi_flush();
 263 }
 264
 265 void __init printk_nmi_init(void)
 266 {
 267         int cpu;
 268
 269         for_each_possible_cpu(cpu) {
 270                 struct nmi_seq_buf *s = &per_cpu(nmi_print_seq, cpu);
 271
 272                 init_irq_work(&s->work, __printk_nmi_flush);
 273         }
 274
 275         /* Make sure that IRQ works are initialized before enabling. */
 276         smp_wmb();
 277         printk_nmi_irq_ready = 1;
 278
 279         /* Flush pending messages that did not have scheduled IRQ works. */
 280         printk_nmi_flush();
 281 }
 282
 283 void printk_nmi_enter(void)
 284 {
 285         this_cpu_write(printk_func, vprintk_nmi);
 286 }
 287
 288 void printk_nmi_exit(void)
 289 {
 290         this_cpu_write(printk_func, vprintk_default);
 291 }