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Date:   Fri, 14 Sep 2018 15:57:28 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To:     Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>
Cc:     Alexander Potapenko <glider@google.com>,
        Dmitriy Vyukov <dvyukov@google.com>,
        penguin-kernel@i-love.sakura.ne.jp,
        kbuild test robot <fengguang.wu@intel.com>,
        syzkaller <syzkaller@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Subject: Re: [PATCH] printk: inject caller information into the body of
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On (09/13/18 23:28), Sergey Senozhatsky wrote:
> Not that I see any problems with pr_line_flush(). But can drop it, sure.
> pr_line() is a replacement for pr_cont() and as such it's not for multi-line
> buffering.

OK, attached.
Let me know if anything needs to improved (including broken English).
Will we keep in the printk tree or shall I send a formal patch to Andrew?

===

From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Subject: [PATCH] lib/seq_buf: add pr_line buffering API

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
 include/linux/kern_levels.h |  3 ++
 include/linux/seq_buf.h     | 60 +++++++++++++++++++++++++++++++++++++
 lib/seq_buf.c               | 57 +++++++++++++++++++++++++++++++++++
 3 files changed, 120 insertions(+)

diff --git a/include/linux/kern_levels.h b/include/linux/kern_levels.h
index d237fe854ad9..9c281ac745b3 100644
--- a/include/linux/kern_levels.h
+++ b/include/linux/kern_levels.h
@@ -20,6 +20,9 @@
  * Annotation for a "continued" line of log printout (only done after a
  * line that had no enclosing \n). Only to be used by core/arch code
  * during early bootup (a continued line is not SMP-safe otherwise).
+ *
+ * Please consider pr_line()/vpr_line() functions for SMP-safe continued
+ * line printing.
  */
 #define KERN_CONT	KERN_SOH "c"
 
diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
index aa5deb041c25..b33aeea14803 100644
--- a/include/linux/seq_buf.h
+++ b/include/linux/seq_buf.h
@@ -23,6 +23,62 @@ struct seq_buf {
 	loff_t			readpos;
 };
 
+#define __SEQ_BUF_INITIALIZER(buf, length)			\
+{								\
+	.buffer			= (buf),			\
+	.size			= (length),			\
+	.len			= 0,				\
+	.readpos		= 0,				\
+}
+
+#ifdef CONFIG_PRINTK
+#define __PR_LINE_BUF_SZ	80
+#else
+#define __PR_LINE_BUF_SZ	0
+#endif
+
+/**
+ * pr_line - printk() line buffer structure
+ * @sb:	underlying seq buffer, which holds the data
+ * @level:	printk() log level (KERN_ERR, etc.)
+ */
+struct pr_line {
+	struct seq_buf		sb;
+	char			*level;
+};
+
+/**
+ * DEFINE_PR_LINE - define a new pr_line variable
+ * @lev:	printk() log level
+ * @name:	variable name
+ *
+ * Defines a new pr_line varialbe, which would use an implicit
+ * stack buffer of size __PR_LINE_BUF_SZ.
+ */
+#define DEFINE_PR_LINE(lev, name)				\
+	char		__line_##name[__PR_LINE_BUF_SZ];	\
+	struct pr_line	name = {				\
+		.sb	= __SEQ_BUF_INITIALIZER(__line_##name,	\
+					__PR_LINE_BUF_SZ),	\
+		.level	= lev,					\
+	}
+
+/**
+ * DEFINE_PR_LINE_BUF - define a new pr_line variable
+ * @lev:	printk() log level
+ * @name:	variable name
+ * @buf:	external buffer
+ * @sz:	external buffer size
+ *
+ * Defines a new pr_line variable, which would use an external
+ * buffer for printk line.
+ */
+#define DEFINE_PR_LINE_BUF(lev, name, buf, sz)			\
+	struct pr_line	name = {				\
+		.sb	= __SEQ_BUF_INITIALIZER(buf, (sz)),	\
+		.level	= lev,					\
+	}
+
 static inline void seq_buf_clear(struct seq_buf *s)
 {
 	s->len = 0;
@@ -131,4 +187,8 @@ extern int
 seq_buf_bprintf(struct seq_buf *s, const char *fmt, const u32 *binary);
 #endif
 
+extern __printf(2, 0)
+int vpr_line(struct pr_line *pl, const char *fmt, va_list args);
+extern __printf(2, 3)
+int pr_line(struct pr_line *pl, const char *fmt, ...);
 #endif /* _LINUX_SEQ_BUF_H */
diff --git a/lib/seq_buf.c b/lib/seq_buf.c
index 11f2ae0f9099..fada7623f168 100644
--- a/lib/seq_buf.c
+++ b/lib/seq_buf.c
@@ -324,3 +324,60 @@ int seq_buf_to_user(struct seq_buf *s, char __user *ubuf, int cnt)
 	s->readpos += cnt;
 	return cnt;
 }
+
+/**
+ * vpr_line - Append data to the printk() line buffer
+ * @pl: the pr_line descriptor
+ * @fmt: printf format string
+ * @args: va_list of arguments from a printf() type function
+ *
+ * Writes a vnprintf() format into the printk() pr_line buffer.
+ * Terminating new-line symbol flushes (prints) the buffer.
+ *
+ * Unlike pr_cont() and printk(KERN_CONT), this function is SMP-safe
+ * and shall be used for continued line printing.
+ *
+ * Returns zero on success, -1 on overflow.
+ */
+int vpr_line(struct pr_line *pl, const char *fmt, va_list args)
+{
+	struct seq_buf *s = &pl->sb;
+	int ret, len;
+
+	ret = seq_buf_vprintf(s, fmt, args);
+
+	len = seq_buf_used(s);
+	if (len && s->buffer[len - 1] == '\n') {
+		printk("%s%.*s", pl->level ? : KERN_DEFAULT, len, s->buffer);
+		seq_buf_clear(s);
+	}
+
+	return ret;
+}
+EXPORT_SYMBOL(vpr_line);
+
+/**
+ * pr_line - Append data to the printk() line buffer
+ * @pl: the pr_line descriptor
+ * @fmt: printf format string
+ *
+ * Writes a printf() format into the printk() pr_line buffer.
+ * Terminating new-line symbol flushes (prints) the buffer.
+ *
+ * Unlike pr_cont() and printk(KERN_CONT), this function is SMP-safe
+ * and shall be used for continued line printing.
+ *
+ * Returns zero on success, -1 on overflow.
+ */
+int pr_line(struct pr_line *pl, const char *fmt, ...)
+{
+	va_list ap;
+	int ret;
+
+	va_start(ap, fmt);
+	ret = vpr_line(pl, fmt, ap);
+	va_end(ap);
+
+	return ret;
+}
+EXPORT_SYMBOL(pr_line);
-- 
2.19.0