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         va_list ap;
  67
  68 again:
  69         len = atomic_read(&s->len);
  70
  71         if (len >= sizeof(s->buffer)) {
  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         va_copy(ap, args);
  84         add = vsnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, ap);
  85         va_end(ap);
  86
  87         /*
  88          * Do it once again if the buffer has been flushed in the meantime.
  89          * Note that atomic_cmpxchg() is an implicit memory barrier that
  90          * makes sure that the data were written before updating s->len.
  91          */
  92         if (atomic_cmpxchg(&s->len, len, len + add) != len)
  93                 goto again;
  94
  95         /* Get flushed in a more safe context. */
  96         if (add && printk_nmi_irq_ready) {
  97                 /* Make sure that IRQ work is really initialized. */
  98                 smp_rmb();
  99                 irq_work_queue(&s->work);
 100         }
 101
 102         return add;
 103 }
 104
 105 static void printk_nmi_flush_line(const char *text, int len)
 106 {
 107         /*
 108          * The buffers are flushed in NMI only on panic.  The messages must
 109          * go only into the ring buffer at this stage.  Consoles will get
 110          * explicitly called later when a crashdump is not generated.
 111          */
 112         if (in_nmi())
 113                 printk_deferred("%.*s", len, text);
 114         else
 115                 printk("%.*s", len, text);
 116
 117 }
 118
 119 /*
 120  * printk one line from the temporary buffer from @start index until
 121  * and including the @end index.
 122  */
 123 static void printk_nmi_flush_seq_line(struct nmi_seq_buf *s,
 124                                         int start, int end)
 125 {
 126         const char *buf = s->buffer + start;
 127
 128         printk_nmi_flush_line(buf, (end - start) + 1);
 129 }
 130
 131 /*
 132  * Flush data from the associated per_CPU buffer. The function
 133  * can be called either via IRQ work or independently.
 134  */
 135 static void __printk_nmi_flush(struct irq_work *work)
 136 {
 137         static raw_spinlock_t read_lock =
 138                 __RAW_SPIN_LOCK_INITIALIZER(read_lock);
 139         struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work);
 140         unsigned long flags;
 141         size_t len, size;
 142         int i, last_i;
 143
 144         /*
 145          * The lock has two functions. First, one reader has to flush all
 146          * available message to make the lockless synchronization with
 147          * writers easier. Second, we do not want to mix messages from
 148          * different CPUs. This is especially important when printing
 149          * a backtrace.
 150          */
 151         raw_spin_lock_irqsave(&read_lock, flags);
 152
 153         i = 0;
 154 more:
 155         len = atomic_read(&s->len);
 156
 157         /*
 158          * This is just a paranoid check that nobody has manipulated
 159          * the buffer an unexpected way. If we printed something then
 160          * @len must only increase.
 161          */
 162         if (i && i >= len) {
 163                 const char *msg = "printk_nmi_flush: internal error\n";
 164
 165                 printk_nmi_flush_line(msg, strlen(msg));
 166         }
 167
 168         if (!len)
 169                 goto out; /* Someone else has already flushed the buffer. */
 170
 171         /* Make sure that data has been written up to the @len */
 172         smp_rmb();
 173
 174         size = min(len, sizeof(s->buffer));
 175         last_i = i;
 176
 177         /* Print line by line. */
 178         for (; i < size; i++) {
 179                 if (s->buffer[i] == '\n') {
 180                         printk_nmi_flush_seq_line(s, last_i, i);
 181                         last_i = i + 1;
 182                 }
 183         }
 184         /* Check if there was a partial line. */
 185         if (last_i < size) {
 186                 printk_nmi_flush_seq_line(s, last_i, size - 1);
 187                 printk_nmi_flush_line("\n", strlen("\n"));
 188         }
 189
 190         /*
 191          * Check that nothing has got added in the meantime and truncate
 192          * the buffer. Note that atomic_cmpxchg() is an implicit memory
 193          * barrier that makes sure that the data were copied before
 194          * updating s->len.
 195          */
 196         if (atomic_cmpxchg(&s->len, len, 0) != len)
 197                 goto more;
 198
 199 out:
 200         raw_spin_unlock_irqrestore(&read_lock, flags);
 201 }
 202
 203 /**
 204  * printk_nmi_flush - flush all per-cpu nmi buffers.
 205  *
 206  * The buffers are flushed automatically via IRQ work. This function
 207  * is useful only when someone wants to be sure that all buffers have
 208  * been flushed at some point.
 209  */
 210 void printk_nmi_flush(void)
 211 {
 212         int cpu;
 213
 214         for_each_possible_cpu(cpu)
 215                 __printk_nmi_flush(&per_cpu(nmi_print_seq, cpu).work);
 216 }
 217
 218 /**
 219  * printk_nmi_flush_on_panic - flush all per-cpu nmi buffers when the system
 220  *      goes down.
 221  *
 222  * Similar to printk_nmi_flush() but it can be called even in NMI context when
 223  * the system goes down. It does the best effort to get NMI messages into
 224  * the main ring buffer.
 225  *
 226  * Note that it could try harder when there is only one CPU online.
 227  */
 228 void printk_nmi_flush_on_panic(void)
 229 {
 230         /*
 231          * Make sure that we could access the main ring buffer.
 232          * Do not risk a double release when more CPUs are up.
 233          */
 234         if (in_nmi() && raw_spin_is_locked(&logbuf_lock)) {
 235                 if (num_online_cpus() > 1)
 236                         return;
 237
 238                 debug_locks_off();
 239                 raw_spin_lock_init(&logbuf_lock);
 240         }
 241
 242         printk_nmi_flush();
 243 }
 244
 245 void __init printk_nmi_init(void)
 246 {
 247         int cpu;
 248
 249         for_each_possible_cpu(cpu) {
 250                 struct nmi_seq_buf *s = &per_cpu(nmi_print_seq, cpu);
 251
 252                 init_irq_work(&s->work, __printk_nmi_flush);
 253         }
 254
 255         /* Make sure that IRQ works are initialized before enabling. */
 256         smp_wmb();
 257         printk_nmi_irq_ready = 1;
 258
 259         /* Flush pending messages that did not have scheduled IRQ works. */
 260         printk_nmi_flush();
 261 }
 262
 263 void notrace printk_nmi_enter(void)
 264 {
 265         this_cpu_write(printk_func, vprintk_nmi);
 266 }
 267
 268 void notrace printk_nmi_exit(void)
 269 {
 270         this_cpu_write(printk_func, vprintk_default);
 271 }