2 * printk_safe.c - Safe printk for printk-deadlock-prone contexts
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/debug_locks.h>
21 #include <linux/smp.h>
22 #include <linux/cpumask.h>
23 #include <linux/irq_work.h>
24 #include <linux/printk.h>
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.
34 * The alternative implementation is chosen transparently
35 * by examinig current printk() context mask stored in @printk_context
38 * The implementation allows to flush the strings also from another CPU.
39 * There are situations when we want to make sure that all buffers
40 * were handled or when IRQs are blocked.
42 static int printk_safe_irq_ready;
44 #define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) - \
47 sizeof(struct irq_work))
49 struct printk_safe_seq_buf {
50 atomic_t len; /* length of written data */
51 atomic_t message_lost;
52 struct irq_work work; /* IRQ work that flushes the buffer */
53 unsigned char buffer[SAFE_LOG_BUF_LEN];
56 static DEFINE_PER_CPU(struct printk_safe_seq_buf, safe_print_seq);
57 static DEFINE_PER_CPU(int, printk_context);
59 static DEFINE_RAW_SPINLOCK(safe_read_lock);
61 #ifdef CONFIG_PRINTK_NMI
62 static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq);
65 /* Get flushed in a more safe context. */
66 static void queue_flush_work(struct printk_safe_seq_buf *s)
68 if (printk_safe_irq_ready) {
69 /* Make sure that IRQ work is really initialized. */
71 irq_work_queue(&s->work);
76 * Add a message to per-CPU context-dependent buffer. NMI and printk-safe
77 * have dedicated buffers, because otherwise printk-safe preempted by
78 * NMI-printk would have overwritten the NMI messages.
80 * The messages are fushed from irq work (or from panic()), possibly,
81 * from other CPU, concurrently with printk_safe_log_store(). Should this
82 * happen, printk_safe_log_store() will notice the buffer->len mismatch
83 * and repeat the write.
85 static __printf(2, 0) int printk_safe_log_store(struct printk_safe_seq_buf *s,
86 const char *fmt, va_list args)
93 len = atomic_read(&s->len);
95 /* The trailing '\0' is not counted into len. */
96 if (len >= sizeof(s->buffer) - 1) {
97 atomic_inc(&s->message_lost);
103 * Make sure that all old data have been read before the buffer
104 * was reset. This is not needed when we just append data.
110 add = vscnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, ap);
116 * Do it once again if the buffer has been flushed in the meantime.
117 * Note that atomic_cmpxchg() is an implicit memory barrier that
118 * makes sure that the data were written before updating s->len.
120 if (atomic_cmpxchg(&s->len, len, len + add) != len)
127 static inline void printk_safe_flush_line(const char *text, int len)
130 * Avoid any console drivers calls from here, because we may be
131 * in NMI or printk_safe context (when in panic). The messages
132 * must go only into the ring buffer at this stage. Consoles will
133 * get explicitly called later when a crashdump is not generated.
135 printk_deferred("%.*s", len, text);
138 /* printk part of the temporary buffer line by line */
139 static int printk_safe_flush_buffer(const char *start, size_t len)
148 /* Print line by line. */
151 printk_safe_flush_line(start, c - start + 1);
157 /* Handle continuous lines or missing new line. */
158 if ((c + 1 < end) && printk_get_level(c)) {
160 c = printk_skip_level(c);
164 printk_safe_flush_line(start, c - start);
174 /* Check if there was a partial line. Ignore pure header. */
175 if (start < end && !header) {
176 static const char newline[] = KERN_CONT "\n";
178 printk_safe_flush_line(start, end - start);
179 printk_safe_flush_line(newline, strlen(newline));
185 static void report_message_lost(struct printk_safe_seq_buf *s)
187 int lost = atomic_xchg(&s->message_lost, 0);
190 printk_deferred("Lost %d message(s)!\n", lost);
194 * Flush data from the associated per-CPU buffer. The function
195 * can be called either via IRQ work or independently.
197 static void __printk_safe_flush(struct irq_work *work)
199 struct printk_safe_seq_buf *s =
200 container_of(work, struct printk_safe_seq_buf, work);
206 * The lock has two functions. First, one reader has to flush all
207 * available message to make the lockless synchronization with
208 * writers easier. Second, we do not want to mix messages from
209 * different CPUs. This is especially important when printing
212 raw_spin_lock_irqsave(&safe_read_lock, flags);
216 len = atomic_read(&s->len);
219 * This is just a paranoid check that nobody has manipulated
220 * the buffer an unexpected way. If we printed something then
221 * @len must only increase. Also it should never overflow the
224 if ((i && i >= len) || len > sizeof(s->buffer)) {
225 const char *msg = "printk_safe_flush: internal error\n";
227 printk_safe_flush_line(msg, strlen(msg));
232 goto out; /* Someone else has already flushed the buffer. */
234 /* Make sure that data has been written up to the @len */
236 i += printk_safe_flush_buffer(s->buffer + i, len - i);
239 * Check that nothing has got added in the meantime and truncate
240 * the buffer. Note that atomic_cmpxchg() is an implicit memory
241 * barrier that makes sure that the data were copied before
244 if (atomic_cmpxchg(&s->len, len, 0) != len)
248 report_message_lost(s);
249 raw_spin_unlock_irqrestore(&safe_read_lock, flags);
253 * printk_safe_flush - flush all per-cpu nmi buffers.
255 * The buffers are flushed automatically via IRQ work. This function
256 * is useful only when someone wants to be sure that all buffers have
257 * been flushed at some point.
259 void printk_safe_flush(void)
263 for_each_possible_cpu(cpu) {
264 #ifdef CONFIG_PRINTK_NMI
265 __printk_safe_flush(&per_cpu(nmi_print_seq, cpu).work);
267 __printk_safe_flush(&per_cpu(safe_print_seq, cpu).work);
272 * printk_safe_flush_on_panic - flush all per-cpu nmi buffers when the system
275 * Similar to printk_safe_flush() but it can be called even in NMI context when
276 * the system goes down. It does the best effort to get NMI messages into
277 * the main ring buffer.
279 * Note that it could try harder when there is only one CPU online.
281 void printk_safe_flush_on_panic(void)
284 * Make sure that we could access the main ring buffer.
285 * Do not risk a double release when more CPUs are up.
287 if (raw_spin_is_locked(&logbuf_lock)) {
288 if (num_online_cpus() > 1)
292 raw_spin_lock_init(&logbuf_lock);
295 if (raw_spin_is_locked(&safe_read_lock)) {
296 if (num_online_cpus() > 1)
300 raw_spin_lock_init(&safe_read_lock);
306 #ifdef CONFIG_PRINTK_NMI
308 * Safe printk() for NMI context. It uses a per-CPU buffer to
309 * store the message. NMIs are not nested, so there is always only
310 * one writer running. But the buffer might get flushed from another
311 * CPU, so we need to be careful.
313 static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args)
315 struct printk_safe_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
317 return printk_safe_log_store(s, fmt, args);
320 void notrace printk_nmi_enter(void)
322 this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
325 void notrace printk_nmi_exit(void)
327 this_cpu_and(printk_context, ~PRINTK_NMI_CONTEXT_MASK);
331 * Marks a code that might produce many messages in NMI context
332 * and the risk of losing them is more critical than eventual
335 * It has effect only when called in NMI context. Then printk()
336 * will try to store the messages into the main logbuf directly
337 * and use the per-CPU buffers only as a fallback when the lock
340 void printk_nmi_direct_enter(void)
342 if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
343 this_cpu_or(printk_context, PRINTK_NMI_DIRECT_CONTEXT_MASK);
346 void printk_nmi_direct_exit(void)
348 this_cpu_and(printk_context, ~PRINTK_NMI_DIRECT_CONTEXT_MASK);
353 static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args)
358 #endif /* CONFIG_PRINTK_NMI */
361 * Lock-less printk(), to avoid deadlocks should the printk() recurse
362 * into itself. It uses a per-CPU buffer to store the message, just like
365 static __printf(1, 0) int vprintk_safe(const char *fmt, va_list args)
367 struct printk_safe_seq_buf *s = this_cpu_ptr(&safe_print_seq);
369 return printk_safe_log_store(s, fmt, args);
372 /* Can be preempted by NMI. */
373 void __printk_safe_enter(void)
375 this_cpu_inc(printk_context);
378 /* Can be preempted by NMI. */
379 void __printk_safe_exit(void)
381 this_cpu_dec(printk_context);
384 __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
387 * Try to use the main logbuf even in NMI. But avoid calling console
388 * drivers that might have their own locks.
390 if ((this_cpu_read(printk_context) & PRINTK_NMI_DIRECT_CONTEXT_MASK) &&
391 raw_spin_trylock(&logbuf_lock)) {
394 len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, 0, fmt, args);
395 raw_spin_unlock(&logbuf_lock);
396 defer_console_output();
400 /* Use extra buffer in NMI when logbuf_lock is taken or in safe mode. */
401 if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
402 return vprintk_nmi(fmt, args);
404 /* Use extra buffer to prevent a recursion deadlock in safe mode. */
405 if (this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK)
406 return vprintk_safe(fmt, args);
409 return vprintk_default(fmt, args);
412 void __init printk_safe_init(void)
416 for_each_possible_cpu(cpu) {
417 struct printk_safe_seq_buf *s;
419 s = &per_cpu(safe_print_seq, cpu);
420 init_irq_work(&s->work, __printk_safe_flush);
422 #ifdef CONFIG_PRINTK_NMI
423 s = &per_cpu(nmi_print_seq, cpu);
424 init_irq_work(&s->work, __printk_safe_flush);
428 /* Make sure that IRQ works are initialized before enabling. */
430 printk_safe_irq_ready = 1;
432 /* Flush pending messages that did not have scheduled IRQ works. */