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.
43 #define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) - \
46 sizeof(struct irq_work))
48 struct printk_safe_seq_buf {
49 atomic_t len; /* length of written data */
50 atomic_t message_lost;
51 struct irq_work work; /* IRQ work that flushes the buffer */
52 unsigned char buffer[SAFE_LOG_BUF_LEN];
55 static DEFINE_PER_CPU(struct printk_safe_seq_buf, safe_print_seq);
56 static DEFINE_PER_CPU(int, printk_context);
58 static DEFINE_RAW_SPINLOCK(safe_read_lock);
60 #ifdef CONFIG_PRINTK_NMI
61 static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq);
64 /* Get flushed in a more safe context. */
65 static void queue_flush_work(struct printk_safe_seq_buf *s)
67 if (printk_percpu_data_ready())
68 irq_work_queue(&s->work);
72 * Add a message to per-CPU context-dependent buffer. NMI and printk-safe
73 * have dedicated buffers, because otherwise printk-safe preempted by
74 * NMI-printk would have overwritten the NMI messages.
76 * The messages are flushed from irq work (or from panic()), possibly,
77 * from other CPU, concurrently with printk_safe_log_store(). Should this
78 * happen, printk_safe_log_store() will notice the buffer->len mismatch
79 * and repeat the write.
81 static __printf(2, 0) int printk_safe_log_store(struct printk_safe_seq_buf *s,
82 const char *fmt, va_list args)
89 len = atomic_read(&s->len);
91 /* The trailing '\0' is not counted into len. */
92 if (len >= sizeof(s->buffer) - 1) {
93 atomic_inc(&s->message_lost);
99 * Make sure that all old data have been read before the buffer
100 * was reset. This is not needed when we just append data.
106 add = vscnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, ap);
112 * Do it once again if the buffer has been flushed in the meantime.
113 * Note that atomic_cmpxchg() is an implicit memory barrier that
114 * makes sure that the data were written before updating s->len.
116 if (atomic_cmpxchg(&s->len, len, len + add) != len)
123 static inline void printk_safe_flush_line(const char *text, int len)
126 * Avoid any console drivers calls from here, because we may be
127 * in NMI or printk_safe context (when in panic). The messages
128 * must go only into the ring buffer at this stage. Consoles will
129 * get explicitly called later when a crashdump is not generated.
131 printk_deferred("%.*s", len, text);
134 /* printk part of the temporary buffer line by line */
135 static int printk_safe_flush_buffer(const char *start, size_t len)
144 /* Print line by line. */
147 printk_safe_flush_line(start, c - start + 1);
153 /* Handle continuous lines or missing new line. */
154 if ((c + 1 < end) && printk_get_level(c)) {
156 c = printk_skip_level(c);
160 printk_safe_flush_line(start, c - start);
170 /* Check if there was a partial line. Ignore pure header. */
171 if (start < end && !header) {
172 static const char newline[] = KERN_CONT "\n";
174 printk_safe_flush_line(start, end - start);
175 printk_safe_flush_line(newline, strlen(newline));
181 static void report_message_lost(struct printk_safe_seq_buf *s)
183 int lost = atomic_xchg(&s->message_lost, 0);
186 printk_deferred("Lost %d message(s)!\n", lost);
190 * Flush data from the associated per-CPU buffer. The function
191 * can be called either via IRQ work or independently.
193 static void __printk_safe_flush(struct irq_work *work)
195 struct printk_safe_seq_buf *s =
196 container_of(work, struct printk_safe_seq_buf, work);
202 * The lock has two functions. First, one reader has to flush all
203 * available message to make the lockless synchronization with
204 * writers easier. Second, we do not want to mix messages from
205 * different CPUs. This is especially important when printing
208 raw_spin_lock_irqsave(&safe_read_lock, flags);
212 len = atomic_read(&s->len);
215 * This is just a paranoid check that nobody has manipulated
216 * the buffer an unexpected way. If we printed something then
217 * @len must only increase. Also it should never overflow the
220 if ((i && i >= len) || len > sizeof(s->buffer)) {
221 const char *msg = "printk_safe_flush: internal error\n";
223 printk_safe_flush_line(msg, strlen(msg));
228 goto out; /* Someone else has already flushed the buffer. */
230 /* Make sure that data has been written up to the @len */
232 i += printk_safe_flush_buffer(s->buffer + i, len - i);
235 * Check that nothing has got added in the meantime and truncate
236 * the buffer. Note that atomic_cmpxchg() is an implicit memory
237 * barrier that makes sure that the data were copied before
240 if (atomic_cmpxchg(&s->len, len, 0) != len)
244 report_message_lost(s);
245 raw_spin_unlock_irqrestore(&safe_read_lock, flags);
249 * printk_safe_flush - flush all per-cpu nmi buffers.
251 * The buffers are flushed automatically via IRQ work. This function
252 * is useful only when someone wants to be sure that all buffers have
253 * been flushed at some point.
255 void printk_safe_flush(void)
259 for_each_possible_cpu(cpu) {
260 #ifdef CONFIG_PRINTK_NMI
261 __printk_safe_flush(&per_cpu(nmi_print_seq, cpu).work);
263 __printk_safe_flush(&per_cpu(safe_print_seq, cpu).work);
268 * printk_safe_flush_on_panic - flush all per-cpu nmi buffers when the system
271 * Similar to printk_safe_flush() but it can be called even in NMI context when
272 * the system goes down. It does the best effort to get NMI messages into
273 * the main ring buffer.
275 * Note that it could try harder when there is only one CPU online.
277 void printk_safe_flush_on_panic(void)
280 * Make sure that we could access the main ring buffer.
281 * Do not risk a double release when more CPUs are up.
283 if (raw_spin_is_locked(&logbuf_lock)) {
284 if (num_online_cpus() > 1)
288 raw_spin_lock_init(&logbuf_lock);
291 if (raw_spin_is_locked(&safe_read_lock)) {
292 if (num_online_cpus() > 1)
296 raw_spin_lock_init(&safe_read_lock);
302 #ifdef CONFIG_PRINTK_NMI
304 * Safe printk() for NMI context. It uses a per-CPU buffer to
305 * store the message. NMIs are not nested, so there is always only
306 * one writer running. But the buffer might get flushed from another
307 * CPU, so we need to be careful.
309 static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args)
311 struct printk_safe_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
313 return printk_safe_log_store(s, fmt, args);
316 void notrace printk_nmi_enter(void)
318 this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
321 void notrace printk_nmi_exit(void)
323 this_cpu_and(printk_context, ~PRINTK_NMI_CONTEXT_MASK);
327 * Marks a code that might produce many messages in NMI context
328 * and the risk of losing them is more critical than eventual
331 * It has effect only when called in NMI context. Then printk()
332 * will try to store the messages into the main logbuf directly
333 * and use the per-CPU buffers only as a fallback when the lock
336 void printk_nmi_direct_enter(void)
338 if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
339 this_cpu_or(printk_context, PRINTK_NMI_DIRECT_CONTEXT_MASK);
342 void printk_nmi_direct_exit(void)
344 this_cpu_and(printk_context, ~PRINTK_NMI_DIRECT_CONTEXT_MASK);
349 static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args)
354 #endif /* CONFIG_PRINTK_NMI */
357 * Lock-less printk(), to avoid deadlocks should the printk() recurse
358 * into itself. It uses a per-CPU buffer to store the message, just like
361 static __printf(1, 0) int vprintk_safe(const char *fmt, va_list args)
363 struct printk_safe_seq_buf *s = this_cpu_ptr(&safe_print_seq);
365 return printk_safe_log_store(s, fmt, args);
368 /* Can be preempted by NMI. */
369 void __printk_safe_enter(void)
371 this_cpu_inc(printk_context);
374 /* Can be preempted by NMI. */
375 void __printk_safe_exit(void)
377 this_cpu_dec(printk_context);
380 __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
383 * Try to use the main logbuf even in NMI. But avoid calling console
384 * drivers that might have their own locks.
386 if ((this_cpu_read(printk_context) & PRINTK_NMI_DIRECT_CONTEXT_MASK) &&
387 raw_spin_trylock(&logbuf_lock)) {
390 len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, 0, fmt, args);
391 raw_spin_unlock(&logbuf_lock);
392 defer_console_output();
396 /* Use extra buffer in NMI when logbuf_lock is taken or in safe mode. */
397 if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
398 return vprintk_nmi(fmt, args);
400 /* Use extra buffer to prevent a recursion deadlock in safe mode. */
401 if (this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK)
402 return vprintk_safe(fmt, args);
405 return vprintk_default(fmt, args);
408 void __init printk_safe_init(void)
412 for_each_possible_cpu(cpu) {
413 struct printk_safe_seq_buf *s;
415 s = &per_cpu(safe_print_seq, cpu);
416 init_irq_work(&s->work, __printk_safe_flush);
418 #ifdef CONFIG_PRINTK_NMI
419 s = &per_cpu(nmi_print_seq, cpu);
420 init_irq_work(&s->work, __printk_safe_flush);
424 /* Flush pending messages that did not have scheduled IRQ works. */