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From:   John Ogness <john.ogness@linutronix.de>
To:     Petr Mladek <pmladek@suse.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org
Subject: [PATCH next v2 3/3] printk: remove logbuf_lock, add syslog_lock
Date:   Tue,  1 Dec 2020 21:59:41 +0106
Message-Id: <20201201205341.3871-4-john.ogness@linutronix.de>
In-Reply-To: <20201201205341.3871-1-john.ogness@linutronix.de>
References: <20201201205341.3871-1-john.ogness@linutronix.de>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

Since the ringbuffer is lockless, there is no need for it to be
protected by @logbuf_lock. Remove @logbuf_lock.

This means that printk_nmi_direct and printk_safe_flush_on_panic()
no longer need to acquire any lock to run.

The global variables @syslog_seq, @syslog_partial, @syslog_time
were also protected by @logbuf_lock. Introduce @syslog_lock to
protect these.

@console_seq, @exclusive_console_stop_seq, @console_dropped are
protected by @console_lock.

Without @logbuf_lock it is no longer possible to use the single
static buffer for temporarily sprint'ing the message. Instead,
use vsnprintf() to determine the length and perform the real
vscnprintf() using the area reserved from the ringbuffer. This
leads to suboptimal packing of the message data, but will
result in less wasted storage than multiple per-cpu buffers to
support lockless temporary sprint'ing.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 include/linux/printk.h      |   1 +
 kernel/printk/internal.h    |   4 +-
 kernel/printk/printk.c      | 285 +++++++++++++++++++-----------------
 kernel/printk/printk_safe.c |  18 +--
 4 files changed, 157 insertions(+), 151 deletions(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index fe7eb2351610..6d8f844bfdff 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -45,6 +45,7 @@ static inline const char *printk_skip_headers(const char *buffer)
 }
 
 #define CONSOLE_EXT_LOG_MAX	8192
+#define CONSOLE_LOG_MAX		1024
 
 /* printk's without a loglevel use this.. */
 #define MESSAGE_LOGLEVEL_DEFAULT CONFIG_MESSAGE_LOGLEVEL_DEFAULT
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 3a8fd491758c..e7acc2888c8e 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -12,8 +12,6 @@
 
 #define PRINTK_NMI_CONTEXT_OFFSET	0x010000000
 
-extern raw_spinlock_t logbuf_lock;
-
 __printf(4, 0)
 int vprintk_store(int facility, int level,
 		  const struct dev_printk_info *dev_info,
@@ -59,7 +57,7 @@ void defer_console_output(void);
 __printf(1, 0) int vprintk_func(const char *fmt, va_list args) { return 0; }
 
 /*
- * In !PRINTK builds we still export logbuf_lock spin_lock, console_sem
+ * In !PRINTK builds we still export console_sem
  * semaphore and some of console functions (console_unlock()/etc.), so
  * printk-safe must preserve the existing local IRQ guarantees.
  */
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e9018c4e1b66..7385101210be 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -355,48 +355,18 @@ enum log_flags {
 	LOG_CONT	= 8,	/* text is a fragment of a continuation line */
 };
 
-/*
- * The logbuf_lock protects kmsg buffer, indices, counters.  This can be taken
- * within the scheduler's rq lock. It must be released before calling
- * console_unlock() or anything else that might wake up a process.
- */
-DEFINE_RAW_SPINLOCK(logbuf_lock);
-
-/*
- * Helper macros to lock/unlock logbuf_lock and switch between
- * printk-safe/unsafe modes.
- */
-#define logbuf_lock_irq()				\
-	do {						\
-		printk_safe_enter_irq();		\
-		raw_spin_lock(&logbuf_lock);		\
-	} while (0)
-
-#define logbuf_unlock_irq()				\
-	do {						\
-		raw_spin_unlock(&logbuf_lock);		\
-		printk_safe_exit_irq();			\
-	} while (0)
-
-#define logbuf_lock_irqsave(flags)			\
-	do {						\
-		printk_safe_enter_irqsave(flags);	\
-		raw_spin_lock(&logbuf_lock);		\
-	} while (0)
-
-#define logbuf_unlock_irqrestore(flags)		\
-	do {						\
-		raw_spin_unlock(&logbuf_lock);		\
-		printk_safe_exit_irqrestore(flags);	\
-	} while (0)
+/* The syslog_lock protects syslog_* variables. */
+static DEFINE_SPINLOCK(syslog_lock);
 
 #ifdef CONFIG_PRINTK
 DECLARE_WAIT_QUEUE_HEAD(log_wait);
+/* All 3 protected by @syslog_lock. */
 /* the next printk record to read by syslog(READ) or /proc/kmsg */
 static u64 syslog_seq;
 static size_t syslog_partial;
 static bool syslog_time;
 
+/* All 3 protected by @console_sem. */
 /* the next printk record to write to the console */
 static u64 console_seq;
 static u64 exclusive_console_stop_seq;
@@ -410,7 +380,7 @@ static atomic64_t clear_seq = ATOMIC64_INIT(0);
 #else
 #define PREFIX_MAX		32
 #endif
-#define LOG_LINE_MAX		(1024 - PREFIX_MAX)
+#define LOG_LINE_MAX		(CONSOLE_LOG_MAX - PREFIX_MAX)
 
 #define LOG_LEVEL(v)		((v) & 0x07)
 #define LOG_FACILITY(v)		((v) >> 3 & 0xff)
@@ -720,27 +690,22 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
 	if (ret)
 		return ret;
 
-	logbuf_lock_irq();
 	if (!prb_read_valid(prb, user->seq, r)) {
 		if (file->f_flags & O_NONBLOCK) {
 			ret = -EAGAIN;
-			logbuf_unlock_irq();
 			goto out;
 		}
 
-		logbuf_unlock_irq();
 		ret = wait_event_interruptible(log_wait,
 					prb_read_valid(prb, user->seq, r));
 		if (ret)
 			goto out;
-		logbuf_lock_irq();
 	}
 
 	if (user->seq < prb_first_valid_seq(prb)) {
 		/* our last seen message is gone, return error and reset */
 		user->seq = prb_first_valid_seq(prb);
 		ret = -EPIPE;
-		logbuf_unlock_irq();
 		goto out;
 	}
 
@@ -750,7 +715,6 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
 				  &r->info->dev_info);
 
 	user->seq = r->info->seq + 1;
-	logbuf_unlock_irq();
 
 	if (len > count) {
 		ret = -EINVAL;
@@ -785,7 +749,6 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
 	if (offset)
 		return -ESPIPE;
 
-	logbuf_lock_irq();
 	switch (whence) {
 	case SEEK_SET:
 		/* the first record */
@@ -806,7 +769,6 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
 	default:
 		ret = -EINVAL;
 	}
-	logbuf_unlock_irq();
 	return ret;
 }
 
@@ -820,7 +782,6 @@ static __poll_t devkmsg_poll(struct file *file, poll_table *wait)
 
 	poll_wait(file, &log_wait, wait);
 
-	logbuf_lock_irq();
 	if (prb_read_valid(prb, user->seq, NULL)) {
 		/* return error when data has vanished underneath us */
 		if (user->seq < prb_first_valid_seq(prb))
@@ -828,7 +789,6 @@ static __poll_t devkmsg_poll(struct file *file, poll_table *wait)
 		else
 			ret = EPOLLIN|EPOLLRDNORM;
 	}
-	logbuf_unlock_irq();
 
 	return ret;
 }
@@ -861,9 +821,7 @@ static int devkmsg_open(struct inode *inode, struct file *file)
 	prb_rec_init_rd(&user->record, &user->info,
 			&user->text_buf[0], sizeof(user->text_buf));
 
-	logbuf_lock_irq();
 	user->seq = prb_first_valid_seq(prb);
-	logbuf_unlock_irq();
 
 	file->private_data = user;
 	return 0;
@@ -1071,7 +1029,6 @@ void __init setup_log_buf(int early)
 	struct printk_record r;
 	size_t new_descs_size;
 	size_t new_infos_size;
-	unsigned long flags;
 	char *new_log_buf;
 	unsigned int free;
 	u64 seq;
@@ -1129,8 +1086,6 @@ void __init setup_log_buf(int early)
 		 new_descs, ilog2(new_descs_count),
 		 new_infos);
 
-	logbuf_lock_irqsave(flags);
-
 	log_buf_len = new_log_buf_len;
 	log_buf = new_log_buf;
 	new_log_buf_len = 0;
@@ -1146,8 +1101,6 @@ void __init setup_log_buf(int early)
 	 */
 	prb = &printk_rb_dynamic;
 
-	logbuf_unlock_irqrestore(flags);
-
 	if (seq != prb_next_seq(&printk_rb_static)) {
 		pr_err("dropped %llu messages\n",
 		       prb_next_seq(&printk_rb_static) - seq);
@@ -1423,9 +1376,9 @@ static int syslog_print(char __user *buf, int size)
 		size_t n;
 		size_t skip;
 
-		logbuf_lock_irq();
+		spin_lock_irq(&syslog_lock);
 		if (!prb_read_valid(prb, syslog_seq, &r)) {
-			logbuf_unlock_irq();
+			spin_unlock_irq(&syslog_lock);
 			break;
 		}
 		if (r.info->seq != syslog_seq) {
@@ -1454,7 +1407,7 @@ static int syslog_print(char __user *buf, int size)
 			syslog_partial += n;
 		} else
 			n = 0;
-		logbuf_unlock_irq();
+		spin_unlock_irq(&syslog_lock);
 
 		if (!n)
 			break;
@@ -1479,6 +1432,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
 	struct printk_info info;
 	unsigned int line_count;
 	struct printk_record r;
+	u64 newest_seq;
 	u64 clr_seq;
 	char *text;
 	int len = 0;
@@ -1490,19 +1444,30 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
 		return -ENOMEM;
 
 	time = printk_time;
-	logbuf_lock_irq();
 	clr_seq = atomic64_read(&clear_seq);
 
 	/*
 	 * Find first record that fits, including all following records,
 	 * into the user-provided buffer for this dump.
 	 */
+
 	prb_for_each_info(clr_seq, prb, seq, &info, &line_count)
 		len += get_record_print_text_size(&info, line_count, true, time);
 
-	/* move first record forward until length fits into the buffer */
+	/*
+	 * Keep track of the latest in case new records are coming in fast
+	 * and overwriting the older records.
+	 */
+	newest_seq = seq;
+
+	/*
+	 * Move first record forward until length fits into the buffer. This
+	 * is a best effort attempt. If @newest_seq is reached because the
+	 * ringbuffer is wrapping too fast, just start filling the buffer
+	 * from there.
+	 */
 	prb_for_each_info(clr_seq, prb, seq, &info, &line_count) {
-		if (len <= size)
+		if (len <= size || info.seq > newest_seq)
 			break;
 		len -= get_record_print_text_size(&info, line_count, true, time);
 	}
@@ -1520,12 +1485,10 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
 			break;
 		}
 
-		logbuf_unlock_irq();
 		if (copy_to_user(buf + len, text, textlen))
 			len = -EFAULT;
 		else
 			len += textlen;
-		logbuf_lock_irq();
 
 		if (len < 0)
 			break;
@@ -1533,7 +1496,6 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
 
 	if (clear)
 		atomic64_set(&clear_seq, seq);
-	logbuf_unlock_irq();
 
 	kfree(text);
 	return len;
@@ -1541,9 +1503,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
 
 static void syslog_clear(void)
 {
-	logbuf_lock_irq();
 	atomic64_set(&clear_seq, prb_next_seq(prb));
-	logbuf_unlock_irq();
 }
 
 int do_syslog(int type, char __user *buf, int len, int source)
@@ -1551,6 +1511,7 @@ int do_syslog(int type, char __user *buf, int len, int source)
 	bool clear = false;
 	static int saved_console_loglevel = LOGLEVEL_DEFAULT;
 	int error;
+	u64 seq;
 
 	error = check_syslog_permissions(type, source);
 	if (error)
@@ -1568,8 +1529,11 @@ int do_syslog(int type, char __user *buf, int len, int source)
 			return 0;
 		if (!access_ok(buf, len))
 			return -EFAULT;
+		spin_lock_irq(&syslog_lock);
+		seq = syslog_seq;
+		spin_unlock_irq(&syslog_lock);
 		error = wait_event_interruptible(log_wait,
-				prb_read_valid(prb, syslog_seq, NULL));
+				prb_read_valid(prb, seq, NULL));
 		if (error)
 			return error;
 		error = syslog_print(buf, len);
@@ -1617,7 +1581,7 @@ int do_syslog(int type, char __user *buf, int len, int source)
 		break;
 	/* Number of chars in the log buffer */
 	case SYSLOG_ACTION_SIZE_UNREAD:
-		logbuf_lock_irq();
+		spin_lock_irq(&syslog_lock);
 		if (syslog_seq < prb_first_valid_seq(prb)) {
 			/* messages are gone, move to first one */
 			syslog_seq = prb_first_valid_seq(prb);
@@ -1644,7 +1608,7 @@ int do_syslog(int type, char __user *buf, int len, int source)
 			}
 			error -= syslog_partial;
 		}
-		logbuf_unlock_irq();
+		spin_unlock_irq(&syslog_lock);
 		break;
 	/* Size of the log buffer */
 	case SYSLOG_ACTION_SIZE_BUFFER:
@@ -1847,6 +1811,65 @@ static void call_console_drivers(const char *ext_text, size_t ext_len,
 	}
 }
 
+#ifdef CONFIG_PRINTK_NMI
+#define NUM_RECURSION_CTX 2
+#else
+#define NUM_RECURSION_CTX 1
+#endif
+
+struct printk_recursion {
+	char	count[NUM_RECURSION_CTX];
+};
+
+static DEFINE_PER_CPU(struct printk_recursion, percpu_printk_recursion);
+static char printk_recursion_count[NUM_RECURSION_CTX];
+
+static char *get_printk_count(void)
+{
+	struct printk_recursion *rec;
+	char *count;
+
+	if (!printk_percpu_data_ready()) {
+		count = &printk_recursion_count[0];
+	} else {
+		rec = this_cpu_ptr(&percpu_printk_recursion);
+
+		count = &rec->count[0];
+	}
+
+#ifdef CONFIG_PRINTK_NMI
+	if (in_nmi())
+		count++;
+#endif
+
+	return count;
+}
+
+static bool printk_enter(unsigned long *flags)
+{
+	char *count;
+
+	local_irq_save(*flags);
+	count = get_printk_count();
+	/* Only 1 level of recursion allowed. */
+	if (*count > 1) {
+		local_irq_restore(*flags);
+		return false;
+	}
+	(*count)++;
+
+	return true;
+}
+
+static void printk_exit(unsigned long flags)
+{
+	char *count;
+
+	count = get_printk_count();
+	(*count)--;
+	local_irq_restore(flags);
+}
+
 int printk_delay_msec __read_mostly;
 
 static inline void printk_delay(void)
@@ -1867,40 +1890,75 @@ static inline u32 printk_caller_id(void)
 		0x80000000 + raw_smp_processor_id();
 }
 
-/* Must be called under logbuf_lock. */
+static u16 printk_sprint(char *text, u16 size, int facility, enum log_flags *lflags,
+			 const char *fmt, va_list args)
+{
+	char *orig_text = text;
+	u16 text_len;
+
+	text_len = vscnprintf(text, size, fmt, args);
+
+	/* Mark and strip a trailing newline. */
+	if (text_len && text[text_len - 1] == '\n') {
+		text_len--;
+		*lflags |= LOG_NEWLINE;
+	}
+
+	/* Strip kernel syslog prefix. */
+	if (facility == 0) {
+		while (text_len >= 2 && printk_get_level(text)) {
+			text_len -= 2;
+			text += 2;
+		}
+
+		if (text != orig_text)
+			memmove(orig_text, text, text_len);
+	}
+
+	return text_len;
+}
+
+__printf(4, 0)
 int vprintk_store(int facility, int level,
 		  const struct dev_printk_info *dev_info,
 		  const char *fmt, va_list args)
 {
 	const u32 caller_id = printk_caller_id();
-	static char textbuf[LOG_LINE_MAX];
 	struct prb_reserved_entry e;
 	enum log_flags lflags = 0;
 	struct printk_record r;
+	unsigned long irqflags;
 	u16 trunc_msg_len = 0;
-	char *text = textbuf;
+	char lvlbuf[8];
+	va_list args2;
 	u16 text_len;
+	int ret = 0;
 	u64 ts_nsec;
 
 	ts_nsec = local_clock();
 
+	if (!printk_enter(&irqflags))
+		return 0;
+
+	va_copy(args2, args);
+
 	/*
 	 * The printf needs to come first; we need the syslog
 	 * prefix which might be passed-in as a parameter.
 	 */
-	text_len = vscnprintf(text, sizeof(textbuf), fmt, args);
+	text_len = vsnprintf(&lvlbuf[0], sizeof(lvlbuf), fmt, args) + 1;
+	if (text_len > CONSOLE_LOG_MAX)
+		text_len = CONSOLE_LOG_MAX;
 
-	/* mark and strip a trailing newline */
-	if (text_len && text[text_len-1] == '\n') {
-		text_len--;
-		lflags |= LOG_NEWLINE;
-	}
-
-	/* strip kernel syslog prefix and extract log level or control flags */
+	/* Extract log level or control flags. */
 	if (facility == 0) {
 		int kern_level;
+		int i;
 
-		while ((kern_level = printk_get_level(text)) != 0) {
+		for (i = 0; i < sizeof(lvlbuf); i += 2) {
+			kern_level = printk_get_level(&lvlbuf[i]);
+			if (!kern_level)
+				break;
 			switch (kern_level) {
 			case '0' ... '7':
 				if (level == LOGLEVEL_DEFAULT)
@@ -1909,9 +1967,6 @@ int vprintk_store(int facility, int level,
 			case 'c':	/* KERN_CONT */
 				lflags |= LOG_CONT;
 			}
-
-			text_len -= 2;
-			text += 2;
 		}
 	}
 
@@ -1924,7 +1979,8 @@ int vprintk_store(int facility, int level,
 	if (lflags & LOG_CONT) {
 		prb_rec_init_wr(&r, text_len);
 		if (prb_reserve_in_last(&e, prb, &r, caller_id, LOG_LINE_MAX)) {
-			memcpy(&r.text_buf[r.info->text_len], text, text_len);
+			text_len = printk_sprint(&r.text_buf[r.info->text_len], text_len,
+						 facility, &lflags, fmt, args2);
 			r.info->text_len += text_len;
 
 			if (lflags & LOG_NEWLINE) {
@@ -1934,7 +1990,8 @@ int vprintk_store(int facility, int level,
 				prb_commit(&e);
 			}
 
-			return text_len;
+			ret = text_len;
+			goto out;
 		}
 	}
 
@@ -1945,11 +2002,11 @@ int vprintk_store(int facility, int level,
 
 		prb_rec_init_wr(&r, text_len + trunc_msg_len);
 		if (!prb_reserve(&e, prb, &r))
-			return 0;
+			goto out;
 	}
 
 	/* fill message */
-	memcpy(&r.text_buf[0], text, text_len);
+	text_len = printk_sprint(&r.text_buf[0], text_len, facility, &lflags, fmt, args2);
 	if (trunc_msg_len)
 		memcpy(&r.text_buf[text_len], trunc_msg, trunc_msg_len);
 	r.info->text_len = text_len + trunc_msg_len;
@@ -1967,7 +2024,11 @@ int vprintk_store(int facility, int level,
 	else
 		prb_final_commit(&e);
 
-	return (text_len + trunc_msg_len);
+	ret = text_len + trunc_msg_len;
+out:
+	va_end(args2);
+	printk_exit(irqflags);
+	return ret;
 }
 
 asmlinkage int vprintk_emit(int facility, int level,
@@ -1976,7 +2037,6 @@ asmlinkage int vprintk_emit(int facility, int level,
 {
 	int printed_len;
 	bool in_sched = false;
-	unsigned long flags;
 
 	/* Suppress unimportant messages after panic happens */
 	if (unlikely(suppress_printk))
@@ -1991,9 +2051,7 @@ asmlinkage int vprintk_emit(int facility, int level,
 	printk_delay();
 
 	/* This stops the holder of console_sem just where we want him */
-	logbuf_lock_irqsave(flags);
 	printed_len = vprintk_store(facility, level, dev_info, fmt, args);
-	logbuf_unlock_irqrestore(flags);
 
 	/* If called from the scheduler, we can not call up(). */
 	if (!in_sched) {
@@ -2432,7 +2490,6 @@ void console_unlock(void)
 		size_t len;
 
 		printk_safe_enter_irqsave(flags);
-		raw_spin_lock(&logbuf_lock);
 skip:
 		if (!prb_read_valid(prb, console_seq, &r))
 			break;
@@ -2476,7 +2533,6 @@ void console_unlock(void)
 				console_msg_format & MSG_FORMAT_SYSLOG,
 				printk_time);
 		console_seq++;
-		raw_spin_unlock(&logbuf_lock);
 
 		/*
 		 * While actively printing out messages, if another printk()
@@ -2503,8 +2559,6 @@ void console_unlock(void)
 
 	console_locked = 0;
 
-	raw_spin_unlock(&logbuf_lock);
-
 	up_console_sem();
 
 	/*
@@ -2513,9 +2567,7 @@ void console_unlock(void)
 	 * there's a new owner and the console_unlock() from them will do the
 	 * flush, no worries.
 	 */
-	raw_spin_lock(&logbuf_lock);
 	retry = prb_read_valid(prb, console_seq, NULL);
-	raw_spin_unlock(&logbuf_lock);
 	printk_safe_exit_irqrestore(flags);
 
 	if (retry && console_trylock())
@@ -2579,13 +2631,8 @@ void console_flush_on_panic(enum con_flush_mode mode)
 	console_trylock();
 	console_may_schedule = 0;
 
-	if (mode == CONSOLE_REPLAY_ALL) {
-		unsigned long flags;
-
-		logbuf_lock_irqsave(flags);
+	if (mode == CONSOLE_REPLAY_ALL)
 		console_seq = prb_first_valid_seq(prb);
-		logbuf_unlock_irqrestore(flags);
-	}
 	console_unlock();
 }
 
@@ -2809,11 +2856,7 @@ void register_console(struct console *newcon)
 		nr_ext_console_drivers++;
 
 	if (newcon->flags & CON_PRINTBUFFER) {
-		/*
-		 * console_unlock(); will print out the buffered messages
-		 * for us.
-		 */
-		logbuf_lock_irqsave(flags);
+		spin_lock_irqsave(&syslog_lock, flags);
 		/*
 		 * We're about to replay the log buffer.  Only do this to the
 		 * just-registered console to avoid excessive message spam to
@@ -2826,7 +2869,7 @@ void register_console(struct console *newcon)
 		exclusive_console = newcon;
 		exclusive_console_stop_seq = console_seq;
 		console_seq = syslog_seq;
-		logbuf_unlock_irqrestore(flags);
+		spin_unlock_irqrestore(&syslog_lock, flags);
 	}
 	console_unlock();
 	console_sysfs_notify();
@@ -3190,7 +3233,6 @@ EXPORT_SYMBOL_GPL(kmsg_dump_reason_str);
 void kmsg_dump(enum kmsg_dump_reason reason)
 {
 	struct kmsg_dumper *dumper;
-	unsigned long flags;
 
 	rcu_read_lock();
 	list_for_each_entry_rcu(dumper, &dump_list, list) {
@@ -3210,10 +3252,7 @@ void kmsg_dump(enum kmsg_dump_reason reason)
 		/* initialize iterator with data about the stored records */
 		dumper->active = true;
 
-		logbuf_lock_irqsave(flags);
-		dumper->cur_seq = atomic64_read(&clear_seq);
-		dumper->next_seq = prb_next_seq(prb);
-		logbuf_unlock_irqrestore(flags);
+		kmsg_dump_rewind_nolock(dumper);
 
 		/* invoke dumper which will iterate over records */
 		dumper->dump(dumper, reason);
@@ -3300,14 +3339,7 @@ bool kmsg_dump_get_line_nolock(struct kmsg_dumper *dumper, bool syslog,
 bool kmsg_dump_get_line(struct kmsg_dumper *dumper, bool syslog,
 			char *line, size_t size, size_t *len)
 {
-	unsigned long flags;
-	bool ret;
-
-	logbuf_lock_irqsave(flags);
-	ret = kmsg_dump_get_line_nolock(dumper, syslog, line, size, len);
-	logbuf_unlock_irqrestore(flags);
-
-	return ret;
+	return kmsg_dump_get_line_nolock(dumper, syslog, line, size, len);
 }
 EXPORT_SYMBOL_GPL(kmsg_dump_get_line);
 
@@ -3336,7 +3368,6 @@ bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
 	struct printk_info info;
 	unsigned int line_count;
 	struct printk_record r;
-	unsigned long flags;
 	u64 seq;
 	u64 next_seq;
 	size_t l = 0;
@@ -3348,17 +3379,14 @@ bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
 	if (!dumper->active || !buf || !size)
 		goto out;
 
-	logbuf_lock_irqsave(flags);
 	if (dumper->cur_seq < prb_first_valid_seq(prb)) {
 		/* messages are gone, move to first available one */
 		dumper->cur_seq = prb_first_valid_seq(prb);
 	}
 
 	/* last entry */
-	if (dumper->cur_seq >= dumper->next_seq) {
-		logbuf_unlock_irqrestore(flags);
+	if (dumper->cur_seq >= dumper->next_seq)
 		goto out;
-	}
 
 	/* calculate length of entire buffer */
 	seq = dumper->cur_seq;
@@ -3398,7 +3426,6 @@ bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
 
 	dumper->next_seq = next_seq;
 	ret = true;
-	logbuf_unlock_irqrestore(flags);
 out:
 	if (len)
 		*len = l;
@@ -3413,8 +3440,6 @@ EXPORT_SYMBOL_GPL(kmsg_dump_get_buffer);
  * Reset the dumper's iterator so that kmsg_dump_get_line() and
  * kmsg_dump_get_buffer() can be called again and used multiple
  * times within the same dumper.dump() callback.
- *
- * The function is similar to kmsg_dump_rewind(), but grabs no locks.
  */
 void kmsg_dump_rewind_nolock(struct kmsg_dumper *dumper)
 {
@@ -3432,11 +3457,7 @@ void kmsg_dump_rewind_nolock(struct kmsg_dumper *dumper)
  */
 void kmsg_dump_rewind(struct kmsg_dumper *dumper)
 {
-	unsigned long flags;
-
-	logbuf_lock_irqsave(flags);
 	kmsg_dump_rewind_nolock(dumper);
-	logbuf_unlock_irqrestore(flags);
 }
 EXPORT_SYMBOL_GPL(kmsg_dump_rewind);
 
diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index a0e6f746de6c..7248b6e3df6c 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -266,18 +266,6 @@ void printk_safe_flush(void)
  */
 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 (raw_spin_is_locked(&logbuf_lock)) {
-		if (num_online_cpus() > 1)
-			return;
-
-		debug_locks_off();
-		raw_spin_lock_init(&logbuf_lock);
-	}
-
 	printk_safe_flush();
 }
 
@@ -371,17 +359,15 @@ __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
 	 * Try to use the main logbuf even in NMI. But avoid calling console
 	 * drivers that might have their own locks.
 	 */
-	if ((this_cpu_read(printk_context) & PRINTK_NMI_DIRECT_CONTEXT_MASK) &&
-	    raw_spin_trylock(&logbuf_lock)) {
+	if ((this_cpu_read(printk_context) & PRINTK_NMI_DIRECT_CONTEXT_MASK)) {
 		int len;
 
 		len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, fmt, args);
-		raw_spin_unlock(&logbuf_lock);
 		defer_console_output();
 		return len;
 	}
 
-	/* Use extra buffer in NMI when logbuf_lock is taken or in safe mode. */
+	/* Use extra buffer in NMI or in safe mode. */
 	if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
 		return vprintk_nmi(fmt, args);
 
-- 
2.20.1