printk: rename nmi.c and exported api

A preparation patch for printk_safe work. No functional change.
- rename nmi.c to print_safe.c
- add `printk_safe' prefix to some (which used both by printk-safe
  and printk-nmi) of the exported functions.

Link: http://lkml.kernel.org/r/20161227141611.940-3-sergey.senozhatsky@gmail.com
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Tejun Heo <tj@kernel.org>
Cc: Calvin Owens <calvinowens@fb.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

include/linux/printk.h

@@ -147,17 +147,17 @@ void early_printk(const char *s, ...) { }
 #endif
 
 #ifdef CONFIG_PRINTK_NMI
-extern void printk_nmi_init(void);
+extern void printk_safe_init(void);
 extern void printk_nmi_enter(void);
 extern void printk_nmi_exit(void);
-extern void printk_nmi_flush(void);
-extern void printk_nmi_flush_on_panic(void);
+extern void printk_safe_flush(void);
+extern void printk_safe_flush_on_panic(void);
 #else
-static inline void printk_nmi_init(void) { }
+static inline void printk_safe_init(void) { }
 static inline void printk_nmi_enter(void) { }
 static inline void printk_nmi_exit(void) { }
-static inline void printk_nmi_flush(void) { }
-static inline void printk_nmi_flush_on_panic(void) { }
+static inline void printk_safe_flush(void) { }
+static inline void printk_safe_flush_on_panic(void) { }
 #endif /* PRINTK_NMI */
 
 #ifdef CONFIG_PRINTK

init/Kconfig

@@ -875,17 +875,19 @@ config LOG_CPU_MAX_BUF_SHIFT
 		     13 =>   8 KB for each CPU
 		     12 =>   4 KB for each CPU
 
-config NMI_LOG_BUF_SHIFT
-	int "Temporary per-CPU NMI log buffer size (12 => 4KB, 13 => 8KB)"
+config PRINTK_SAFE_LOG_BUF_SHIFT
+	int "Temporary per-CPU printk log buffer size (12 => 4KB, 13 => 8KB)"
 	range 10 21
 	default 13
-	depends on PRINTK_NMI
+	depends on PRINTK
 	help
-	  Select the size of a per-CPU buffer where NMI messages are temporary
-	  stored. They are copied to the main log buffer in a safe context
-	  to avoid a deadlock. The value defines the size as a power of 2.
+	  Select the size of an alternate printk per-CPU buffer where messages
+	  printed from usafe contexts are temporary stored. One example would
+	  be NMI messages, another one - printk recursion. The messages are
+	  copied to the main log buffer in a safe context to avoid a deadlock.
+	  The value defines the size as a power of 2.
 
-	  NMI messages are rare and limited. The largest one is when
+	  Those messages are rare and limited. The largest one is when
 	  a backtrace is printed. It usually fits into 4KB. Select
 	  8KB if you want to be on the safe side.
 

init/main.c

@@ -580,7 +580,7 @@ asmlinkage __visible void __init start_kernel(void)
 	timekeeping_init();
 	time_init();
 	sched_clock_postinit();
-	printk_nmi_init();
+	printk_safe_init();
 	perf_event_init();
 	profile_init();
 	call_function_init();

kernel/kexec_core.c

@@ -916,7 +916,7 @@ void crash_kexec(struct pt_regs *regs)
 	old_cpu = atomic_cmpxchg(&panic_cpu, PANIC_CPU_INVALID, this_cpu);
 	if (old_cpu == PANIC_CPU_INVALID) {
 		/* This is the 1st CPU which comes here, so go ahead. */
-		printk_nmi_flush_on_panic();
+		printk_safe_flush_on_panic();
 		__crash_kexec(regs);
 
 		/*

kernel/panic.c

@@ -188,7 +188,7 @@ void panic(const char *fmt, ...)
 	 * Bypass the panic_cpu check and call __crash_kexec directly.
 	 */
 	if (!_crash_kexec_post_notifiers) {
-		printk_nmi_flush_on_panic();
+		printk_safe_flush_on_panic();
 		__crash_kexec(NULL);
 
 		/*
@@ -213,7 +213,7 @@ void panic(const char *fmt, ...)
 	atomic_notifier_call_chain(&panic_notifier_list, 0, buf);
 
 	/* Call flush even twice. It tries harder with a single online CPU */
-	printk_nmi_flush_on_panic();
+	printk_safe_flush_on_panic();
 	kmsg_dump(KMSG_DUMP_PANIC);
 
 	/*

kernel/printk/Makefile

@@ -1,3 +1,3 @@
 obj-y	= printk.o
-obj-$(CONFIG_PRINTK_NMI)		+= nmi.o
+obj-$(CONFIG_PRINTK_NMI)		+= printk_safe.o
 obj-$(CONFIG_A11Y_BRAILLE_CONSOLE)	+= braille.o

kernel/printk/printk_safe.c

@@ -0,0 +1,291 @@
+/*
+ * printk_safe.c - Safe printk in NMI context
+ *
+ * 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
+ * via @printk_func 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.
+ */
+DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
+static int printk_safe_irq_ready;
+atomic_t nmi_message_lost;
+
+#define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) -	\
+			 sizeof(atomic_t) - sizeof(struct irq_work))
+
+struct printk_safe_seq_buf {
+	atomic_t		len;	/* length of written data */
+	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, nmi_print_seq);
+
+/*
+ * 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 int vprintk_nmi(const char *fmt, va_list args)
+{
+	struct printk_safe_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
+	int add = 0;
+	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(&nmi_message_lost);
+		return 0;
+	}
+
+	/*
+	 * Make sure that all old data have been read before the buffer was
+	 * reseted. This is not needed when we just append data.
+	 */
+	if (!len)
+		smp_rmb();
+
+	add = vscnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, args);
+
+	/*
+	 * 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;
+
+	/* Get flushed in a more safe context. */
+	if (add && printk_safe_irq_ready) {
+		/* Make sure that IRQ work is really initialized. */
+		smp_rmb();
+		irq_work_queue(&s->work);
+	}
+
+	return add;
+}
+
+static void printk_safe_flush_line(const char *text, int len)
+{
+	/*
+	 * The buffers are flushed in NMI only on 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.
+	 */
+	if (in_nmi())
+		printk_deferred("%.*s", len, text);
+	else
+		printk("%.*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;
+}
+
+/*
+ * 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:
+	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)
+		__printk_safe_flush(&per_cpu(nmi_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();
+}
+
+void __init printk_safe_init(void)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		struct printk_safe_seq_buf *s = &per_cpu(nmi_print_seq, cpu);
+
+		init_irq_work(&s->work, __printk_safe_flush);
+	}
+
+	/* Make sure that IRQ works are initialized before enabling. */
+	smp_wmb();
+	printk_safe_irq_ready = 1;
+
+	/* Flush pending messages that did not have scheduled IRQ works. */
+	printk_safe_flush();
+}
+
+void printk_nmi_enter(void)
+{
+	this_cpu_write(printk_func, vprintk_nmi);
+}
+
+void printk_nmi_exit(void)
+{
+	this_cpu_write(printk_func, vprintk_default);
+}

lib/nmi_backtrace.c

@@ -77,7 +77,7 @@ void nmi_trigger_cpumask_backtrace(const cpumask_t *mask,
 	 * Force flush any remote buffers that might be stuck in IRQ context
 	 * and therefore could not run their irq_work.
 	 */
-	printk_nmi_flush();
+	printk_safe_flush();
 
 	clear_bit_unlock(0, &backtrace_flag);
 	put_cpu();