summaryrefslogtreecommitdiffstats
path: root/kernel/printk/printk_safe.c
blob: 3e3c2004bb232661bae666e00e4645fad438aee6 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
/*
 * printk_safe.c - Safe printk for printk-deadlock-prone contexts
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, see <http://www.gnu.org/licenses/>.
 */

#include <linux/preempt.h>
#include <linux/spinlock.h>
#include <linux/debug_locks.h>
#include <linux/smp.h>
#include <linux/cpumask.h>
#include <linux/irq_work.h>
#include <linux/printk.h>

#include "internal.h"

/*
 * printk() could not take logbuf_lock in NMI context. Instead,
 * it uses an alternative implementation that temporary stores
 * the strings into a per-CPU buffer. The content of the buffer
 * is later flushed into the main ring buffer via IRQ work.
 *
 * The alternative implementation is chosen transparently
 * by examinig current printk() context mask stored in @printk_context
 * per-CPU variable.
 *
 * The implementation allows to flush the strings also from another CPU.
 * There are situations when we want to make sure that all buffers
 * were handled or when IRQs are blocked.
 */
static int printk_safe_irq_ready __read_mostly;

#define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) -	\
				sizeof(atomic_t) -			\
				sizeof(atomic_t) -			\
				sizeof(struct irq_work))

struct printk_safe_seq_buf {
	atomic_t		len;	/* length of written data */
	atomic_t		message_lost;
	struct irq_work		work;	/* IRQ work that flushes the buffer */
	unsigned char		buffer[SAFE_LOG_BUF_LEN];
};

static DEFINE_PER_CPU(struct printk_safe_seq_buf, safe_print_seq);
static DEFINE_PER_CPU(int, printk_context);

#ifdef CONFIG_PRINTK_NMI
static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq);
#endif

/* Get flushed in a more safe context. */
static void queue_flush_work(struct printk_safe_seq_buf *s)
{
	if (printk_safe_irq_ready)
		irq_work_queue(&s->work);
}

/*
 * Add a message to per-CPU context-dependent buffer. NMI and printk-safe
 * have dedicated buffers, because otherwise printk-safe preempted by
 * NMI-printk would have overwritten the NMI messages.
 *
 * The messages are flushed from irq work (or from panic()), possibly,
 * from other CPU, concurrently with printk_safe_log_store(). Should this
 * happen, printk_safe_log_store() will notice the buffer->len mismatch
 * and repeat the write.
 */
static __printf(2, 0) int printk_safe_log_store(struct printk_safe_seq_buf *s,
						const char *fmt, va_list args)
{
	int add;
	size_t len;

again:
	len = atomic_read(&s->len);

	/* The trailing '\0' is not counted into len. */
	if (len >= sizeof(s->buffer) - 1) {
		atomic_inc(&s->message_lost);
		queue_flush_work(s);
		return 0;
	}

	/*
	 * Make sure that all old data have been read before the buffer
	 * was reset. This is not needed when we just append data.
	 */
	if (!len)
		smp_rmb();

	add = vscnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, args);
	if (!add)
		return 0;

	/*
	 * Do it once again if the buffer has been flushed in the meantime.
	 * Note that atomic_cmpxchg() is an implicit memory barrier that
	 * makes sure that the data were written before updating s->len.
	 */
	if (atomic_cmpxchg(&s->len, len, len + add) != len)
		goto again;

	queue_flush_work(s);
	return add;
}

static inline void printk_safe_flush_line(const char *text, int len)
{
	/*
	 * Avoid any console drivers calls from here, because we may be
	 * in NMI or printk_safe context (when in panic). The messages
	 * must go only into the ring buffer at this stage.  Consoles will
	 * get explicitly called later when a crashdump is not generated.
	 */
	printk_deferred("%.*s", len, text);
}

/* printk part of the temporary buffer line by line */
static int printk_safe_flush_buffer(const char *start, size_t len)
{
	const char *c, *end;
	bool header;

	c = start;
	end = start + len;
	header = true;

	/* Print line by line. */
	while (c < end) {
		if (*c == '\n') {
			printk_safe_flush_line(start, c - start + 1);
			start = ++c;
			header = true;
			continue;
		}

		/* Handle continuous lines or missing new line. */
		if ((c + 1 < end) && printk_get_level(c)) {
			if (header) {
				c = printk_skip_level(c);
				continue;
			}

			printk_safe_flush_line(start, c - start);
			start = c++;
			header = true;
			continue;
		}

		header = false;
		c++;
	}

	/* Check if there was a partial line. Ignore pure header. */
	if (start < end && !header) {
		static const char newline[] = KERN_CONT "\n";

		printk_safe_flush_line(start, end - start);
		printk_safe_flush_line(newline, strlen(newline));
	}

	return len;
}

static void report_message_lost(struct printk_safe_seq_buf *s)
{
	int lost = atomic_xchg(&s->message_lost, 0);

	if (lost)
		printk_deferred("Lost %d message(s)!\n", lost);
}

/*
 * Flush data from the associated per-CPU buffer. The function
 * can be called either via IRQ work or independently.
 */
static void __printk_safe_flush(struct irq_work *work)
{
	static raw_spinlock_t read_lock =
		__RAW_SPIN_LOCK_INITIALIZER(read_lock);
	struct printk_safe_seq_buf *s =
		container_of(work, struct printk_safe_seq_buf, work);
	unsigned long flags;
	size_t len;
	int i;

	/*
	 * The lock has two functions. First, one reader has to flush all
	 * available message to make the lockless synchronization with
	 * writers easier. Second, we do not want to mix messages from
	 * different CPUs. This is especially important when printing
	 * a backtrace.
	 */
	raw_spin_lock_irqsave(&read_lock, flags);

	i = 0;
more:
	len = atomic_read(&s->len);

	/*
	 * This is just a paranoid check that nobody has manipulated
	 * the buffer an unexpected way. If we printed something then
	 * @len must only increase. Also it should never overflow the
	 * buffer size.
	 */
	if ((i && i >= len) || len > sizeof(s->buffer)) {
		const char *msg = "printk_safe_flush: internal error\n";

		printk_safe_flush_line(msg, strlen(msg));
		len = 0;
	}

	if (!len)
		goto out; /* Someone else has already flushed the buffer. */

	/* Make sure that data has been written up to the @len */
	smp_rmb();
	i += printk_safe_flush_buffer(s->buffer + i, len - i);

	/*
	 * Check that nothing has got added in the meantime and truncate
	 * the buffer. Note that atomic_cmpxchg() is an implicit memory
	 * barrier that makes sure that the data were copied before
	 * updating s->len.
	 */
	if (atomic_cmpxchg(&s->len, len, 0) != len)
		goto more;

out:
	report_message_lost(s);
	raw_spin_unlock_irqrestore(&read_lock, flags);
}

/**
 * printk_safe_flush - flush all per-cpu nmi buffers.
 *
 * The buffers are flushed automatically via IRQ work. This function
 * is useful only when someone wants to be sure that all buffers have
 * been flushed at some point.
 */
void printk_safe_flush(void)
{
	int cpu;

	for_each_possible_cpu(cpu) {
#ifdef CONFIG_PRINTK_NMI
		__printk_safe_flush(&per_cpu(nmi_print_seq, cpu).work);
#endif
		__printk_safe_flush(&per_cpu(safe_print_seq, cpu).work);
	}
}

/**
 * printk_safe_flush_on_panic - flush all per-cpu nmi buffers when the system
 *	goes down.
 *
 * Similar to printk_safe_flush() but it can be called even in NMI context when
 * the system goes down. It does the best effort to get NMI messages into
 * the main ring buffer.
 *
 * Note that it could try harder when there is only one CPU online.
 */
void printk_safe_flush_on_panic(void)
{
	/*
	 * Make sure that we could access the main ring buffer.
	 * Do not risk a double release when more CPUs are up.
	 */
	if (in_nmi() && raw_spin_is_locked(&logbuf_lock)) {
		if (num_online_cpus() > 1)
			return;

		debug_locks_off();
		raw_spin_lock_init(&logbuf_lock);
	}

	printk_safe_flush();
}

#ifdef CONFIG_PRINTK_NMI
/*
 * Safe printk() for NMI context. It uses a per-CPU buffer to
 * store the message. NMIs are not nested, so there is always only
 * one writer running. But the buffer might get flushed from another
 * CPU, so we need to be careful.
 */
static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args)
{
	struct printk_safe_seq_buf *s = this_cpu_ptr(&nmi_print_seq);

	return printk_safe_log_store(s, fmt, args);
}

void printk_nmi_enter(void)
{
	/*
	 * The size of the extra per-CPU buffer is limited. Use it only when
	 * the main one is locked. If this CPU is not in the safe context,
	 * the lock must be taken on another CPU and we could wait for it.
	 */
	if ((this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK) &&
	    raw_spin_is_locked(&logbuf_lock)) {
		this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
	} else {
		this_cpu_or(printk_context, PRINTK_NMI_DEFERRED_CONTEXT_MASK);
	}
}

void printk_nmi_exit(void)
{
	this_cpu_and(printk_context,
		     ~(PRINTK_NMI_CONTEXT_MASK |
		       PRINTK_NMI_DEFERRED_CONTEXT_MASK));
}

#else

static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args)
{
	return 0;
}

#endif /* CONFIG_PRINTK_NMI */

/*
 * Lock-less printk(), to avoid deadlocks should the printk() recurse
 * into itself. It uses a per-CPU buffer to store the message, just like
 * NMI.
 */
static __printf(1, 0) int vprintk_safe(const char *fmt, va_list args)
{
	struct printk_safe_seq_buf *s = this_cpu_ptr(&safe_print_seq);

	return printk_safe_log_store(s, fmt, args);
}

/* Can be preempted by NMI. */
void __printk_safe_enter(void)
{
	this_cpu_inc(printk_context);
}

/* Can be preempted by NMI. */
void __printk_safe_exit(void)
{
	this_cpu_dec(printk_context);
}

__printf(1, 0) int vprintk_func(const char *fmt, va_list args)
{
	/* Use extra buffer in NMI when logbuf_lock is taken or in safe mode. */
	if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
		return vprintk_nmi(fmt, args);

	/* Use extra buffer to prevent a recursion deadlock in safe mode. */
	if (this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK)
		return vprintk_safe(fmt, args);

	/*
	 * Use the main logbuf when logbuf_lock is available in NMI.
	 * But avoid calling console drivers that might have their own locks.
	 */
	if (this_cpu_read(printk_context) & PRINTK_NMI_DEFERRED_CONTEXT_MASK)
		return vprintk_deferred(fmt, args);

	/* No obstacles. */
	return vprintk_default(fmt, args);
}

void __init printk_safe_init(void)
{
	int cpu;

	for_each_possible_cpu(cpu) {
		struct printk_safe_seq_buf *s;

		s = &per_cpu(safe_print_seq, cpu);
		init_irq_work(&s->work, __printk_safe_flush);

#ifdef CONFIG_PRINTK_NMI
		s = &per_cpu(nmi_print_seq, cpu);
		init_irq_work(&s->work, __printk_safe_flush);
#endif
	}

	/*
	 * In the highly unlikely event that a NMI were to trigger at
	 * this moment. Make sure IRQ work is set up before this
	 * variable is set.
	 */
	barrier();
	printk_safe_irq_ready = 1;

	/* Flush pending messages that did not have scheduled IRQ works. */
	printk_safe_flush();
}
OpenPOWER on IntegriCloud