[PATCH 0/4] KDB: Fix dmesg command

[PATCH 0/4] KDB: Fix dmesg command

[Anton Vorontsov]

Hi all,

The dmesg command appears to be broken after the printk rework. The old
logic in the kdb code makes no sense in terms of current printk/logging
storage format, and KDB simply hangs forever upon entering 'dmesg'
command.

The first patch revives the command by switching to kmsg_dumper iterator.
As a side-effect, the code is now much more simpler.

A few changes were needed in the printk.c: we needed unlocked variant
of the kmsg_dumper iterator, but these can surely wait for 3.6.

It's probably too late even for the first patch to go to 3.5, but
I'll try to convince otherwise. :-) Here we go:

- The current code is broken for sure, and has no hope to work at
  all. It is a regression;
- The new code works for me, and probably works for everyone else;
- If it compiles (and I urge everyone to compile-test it on your
  setup), it hardly can make things worse.

Thanks!

--
 include/linux/kmsg_dump.h      |   16 +++++++
 kernel/debug/kdb/kdb_main.c    |   91 +++++++++++++++-------------------------
 kernel/debug/kdb/kdb_private.h |    1 -
 kernel/printk.c                |   83 +++++++++++++++++++++++-------------
 4 files changed, 104 insertions(+), 87 deletions(-)

-- 
Anton Vorontsov
Email: cbouatmailru@gmail.com

[PATCH for-3.5 1/4] kdb: Revive dmesg command

[Anton Vorontsov]

dmesg command appears to be broken after the printk rework. The old logic
in kdb code makes no sense in terms of current printk/logging storage
format, and KDB simply hangs forever.

This patch revives the command by switching to kmsg_dumper iterator.

The code is now much more simpler and shorter.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 kernel/debug/kdb/kdb_main.c |   91 ++++++++++++++++---------------------------
 1 file changed, 33 insertions(+), 58 deletions(-)

diff --git a/kernel/debug/kdb/kdb_main.c b/kernel/debug/kdb/kdb_main.c
index 67b847d..df17c93 100644
--- a/kernel/debug/kdb/kdb_main.c
+++ b/kernel/debug/kdb/kdb_main.c
@@ -14,6 +14,7 @@
 #include <linux/ctype.h>
 #include <linux/string.h>
 #include <linux/kernel.h>
+#include <linux/kmsg_dump.h>
 #include <linux/reboot.h>
 #include <linux/sched.h>
 #include <linux/sysrq.h>
@@ -2040,8 +2041,15 @@ static int kdb_env(int argc, const char **argv)
  */
 static int kdb_dmesg(int argc, const char **argv)
 {
-	char *syslog_data[4], *start, *end, c = '\0', *p;
-	int diag, logging, logsize, lines = 0, adjust = 0, n;
+	int diag;
+	int logging;
+	int lines = 0;
+	int adjust = 0;
+	int n = 0;
+	int skip = 0;
+	struct kmsg_dumper dumper = { .active = 1 };
+	size_t len;
+	char buf[201];
 
 	if (argc > 2)
 		return KDB_ARGCOUNT;
@@ -2064,22 +2072,10 @@ static int kdb_dmesg(int argc, const char **argv)
 		kdb_set(2, setargs);
 	}
 
-	/* syslog_data[0,1] physical start, end+1.  syslog_data[2,3]
-	 * logical start, end+1. */
-	kdb_syslog_data(syslog_data);
-	if (syslog_data[2] == syslog_data[3])
-		return 0;
-	logsize = syslog_data[1] - syslog_data[0];
-	start = syslog_data[2];
-	end = syslog_data[3];
-#define KDB_WRAP(p) (((p - syslog_data[0]) % logsize) + syslog_data[0])
-	for (n = 0, p = start; p < end; ++p) {
-		c = *KDB_WRAP(p);
-		if (c == '\n')
-			++n;
-	}
-	if (c != '\n')
-		++n;
+	kmsg_dump_rewind(&dumper);
+	while (kmsg_dump_get_line(&dumper, 1, NULL, 0, NULL))
+		n++;
+
 	if (lines < 0) {
 		if (adjust >= n)
 			kdb_printf("buffer only contains %d lines, nothing "
@@ -2087,21 +2083,11 @@ static int kdb_dmesg(int argc, const char **argv)
 		else if (adjust - lines >= n)
 			kdb_printf("buffer only contains %d lines, last %d "
 				   "lines printed\n", n, n - adjust);
-		if (adjust) {
-			for (; start < end && adjust; ++start) {
-				if (*KDB_WRAP(start) == '\n')
-					--adjust;
-			}
-			if (start < end)
-				++start;
-		}
-		for (p = start; p < end && lines; ++p) {
-			if (*KDB_WRAP(p) == '\n')
-				++lines;
-		}
-		end = p;
+		skip = adjust;
+		lines = abs(lines);
 	} else if (lines > 0) {
-		int skip = n - (adjust + lines);
+		skip = n - lines - adjust;
+		lines = abs(lines);
 		if (adjust >= n) {
 			kdb_printf("buffer only contains %d lines, "
 				   "nothing printed\n", n);
@@ -2112,35 +2098,24 @@ static int kdb_dmesg(int argc, const char **argv)
 			kdb_printf("buffer only contains %d lines, first "
 				   "%d lines printed\n", n, lines);
 		}
-		for (; start < end && skip; ++start) {
-			if (*KDB_WRAP(start) == '\n')
-				--skip;
-		}
-		for (p = start; p < end && lines; ++p) {
-			if (*KDB_WRAP(p) == '\n')
-				--lines;
-		}
-		end = p;
+	} else {
+		lines = n;
 	}
-	/* Do a line at a time (max 200 chars) to reduce protocol overhead */
-	c = '\n';
-	while (start != end) {
-		char buf[201];
-		p = buf;
-		if (KDB_FLAG(CMD_INTERRUPT))
-			return 0;
-		while (start < end && (c = *KDB_WRAP(start)) &&
-		       (p - buf) < sizeof(buf)-1) {
-			++start;
-			*p++ = c;
-			if (c == '\n')
-				break;
+
+	if (skip >= n || skip < 0)
+		return 0;
+
+	kmsg_dump_rewind(&dumper);
+	while (kmsg_dump_get_line(&dumper, 1, buf, sizeof(buf), &len)) {
+		if (skip) {
+			skip--;
+			continue;
 		}
-		*p = '\0';
-		kdb_printf("%s", buf);
+		if (!lines--)
+			break;
+
+		kdb_printf("%.*s\n", (int)len - 1, buf);
 	}
-	if (c != '\n')
-		kdb_printf("\n");
 
 	return 0;
 }
-- 
1.7.10.4

[PATCH 2/4] printk: Remove kdb_syslog_data

[Anton Vorontsov]

The function is no longer needed, so remove it.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 kernel/debug/kdb/kdb_private.h |    1 -
 kernel/printk.c                |   15 ---------------
 2 files changed, 16 deletions(-)

diff --git a/kernel/debug/kdb/kdb_private.h b/kernel/debug/kdb/kdb_private.h
index 47c4e56..392ec6a 100644
--- a/kernel/debug/kdb/kdb_private.h
+++ b/kernel/debug/kdb/kdb_private.h
@@ -205,7 +205,6 @@ extern char kdb_grep_string[];
 extern int kdb_grep_leading;
 extern int kdb_grep_trailing;
 extern char *kdb_cmds[];
-extern void kdb_syslog_data(char *syslog_data[]);
 extern unsigned long kdb_task_state_string(const char *);
 extern char kdb_task_state_char (const struct task_struct *);
 extern unsigned long kdb_task_state(const struct task_struct *p,
diff --git a/kernel/printk.c b/kernel/printk.c
index 177fa49..c812967 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -1192,21 +1192,6 @@ SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
 	return do_syslog(type, buf, len, SYSLOG_FROM_CALL);
 }
 
-#ifdef	CONFIG_KGDB_KDB
-/* kdb dmesg command needs access to the syslog buffer.  do_syslog()
- * uses locks so it cannot be used during debugging.  Just tell kdb
- * where the start and end of the physical and logical logs are.  This
- * is equivalent to do_syslog(3).
- */
-void kdb_syslog_data(char *syslog_data[4])
-{
-	syslog_data[0] = log_buf;
-	syslog_data[1] = log_buf + log_buf_len;
-	syslog_data[2] = log_buf + log_first_idx;
-	syslog_data[3] = log_buf + log_next_idx;
-}
-#endif	/* CONFIG_KGDB_KDB */
-
 static bool __read_mostly ignore_loglevel;
 
 static int __init ignore_loglevel_setup(char *str)
-- 
1.7.10.4

[PATCH 3/4] printk: Implement some unlocked kmsg_dump functions

[Anton Vorontsov]

If used from KDB, the locked variants are prone to deadlocks (suppose we
got to the debugger w/ the logbuf lock held).

So, we have to implement a few routines that grab no logbuf lock.

Yet we don't need these functions in modules, so we don't export them.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 include/linux/kmsg_dump.h |   16 +++++++++++
 kernel/printk.c           |   68 ++++++++++++++++++++++++++++++++++++---------
 2 files changed, 71 insertions(+), 13 deletions(-)

diff --git a/include/linux/kmsg_dump.h b/include/linux/kmsg_dump.h
index d6bd501..2e7a1e0 100644
--- a/include/linux/kmsg_dump.h
+++ b/include/linux/kmsg_dump.h
@@ -55,12 +55,17 @@ struct kmsg_dumper {
 #ifdef CONFIG_PRINTK
 void kmsg_dump(enum kmsg_dump_reason reason);
 
+bool kmsg_dump_get_line_nolock(struct kmsg_dumper *dumper, bool syslog,
+			       char *line, size_t size, size_t *len);
+
 bool kmsg_dump_get_line(struct kmsg_dumper *dumper, bool syslog,
 			char *line, size_t size, size_t *len);
 
 bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
 			  char *buf, size_t size, size_t *len);
 
+void kmsg_dump_rewind_nolock(struct kmsg_dumper *dumper);
+
 void kmsg_dump_rewind(struct kmsg_dumper *dumper);
 
 int kmsg_dump_register(struct kmsg_dumper *dumper);
@@ -71,6 +76,13 @@ static inline void kmsg_dump(enum kmsg_dump_reason reason)
 {
 }
 
+static inline bool kmsg_dump_get_line_nolock(struct kmsg_dumper *dumper,
+					     bool syslog, const char *line,
+					     size_t size, size_t *len)
+{
+	return false;
+}
+
 static inline bool kmsg_dump_get_line(struct kmsg_dumper *dumper, bool syslog,
 				const char *line, size_t size, size_t *len)
 {
@@ -83,6 +95,10 @@ static inline bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
 	return false;
 }
 
+static inline void kmsg_dump_rewind_nolock(struct kmsg_dumper *dumper)
+{
+}
+
 static inline void kmsg_dump_rewind(struct kmsg_dumper *dumper)
 {
 }
diff --git a/kernel/printk.c b/kernel/printk.c
index c812967..ac4bc9e 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -2510,7 +2510,7 @@ void kmsg_dump(enum kmsg_dump_reason reason)
 }
 
 /**
- * kmsg_dump_get_line - retrieve one kmsg log line
+ * kmsg_dump_get_line_nolock - retrieve one kmsg log line (unlocked version)
  * @dumper: registered kmsg dumper
  * @syslog: include the "<4>" prefixes
  * @line: buffer to copy the line to
@@ -2525,11 +2525,12 @@ void kmsg_dump(enum kmsg_dump_reason reason)
  *
  * A return value of FALSE indicates that there are no more records to
  * read.
+ *
+ * The function is similar to kmsg_dump_get_line(), but grabs no locks.
  */
-bool kmsg_dump_get_line(struct kmsg_dumper *dumper, bool syslog,
-			char *line, size_t size, size_t *len)
+bool kmsg_dump_get_line_nolock(struct kmsg_dumper *dumper, bool syslog,
+			       char *line, size_t size, size_t *len)
 {
-	unsigned long flags;
 	struct log *msg;
 	size_t l = 0;
 	bool ret = false;
@@ -2537,7 +2538,6 @@ bool kmsg_dump_get_line(struct kmsg_dumper *dumper, bool syslog,
 	if (!dumper->active)
 		goto out;
 
-	raw_spin_lock_irqsave(&logbuf_lock, flags);
 	if (dumper->cur_seq < log_first_seq) {
 		/* messages are gone, move to first available one */
 		dumper->cur_seq = log_first_seq;
@@ -2545,10 +2545,8 @@ bool kmsg_dump_get_line(struct kmsg_dumper *dumper, bool syslog,
 	}
 
 	/* last entry */
-	if (dumper->cur_seq >= log_next_seq) {
-		raw_spin_unlock_irqrestore(&logbuf_lock, flags);
+	if (dumper->cur_seq >= log_next_seq)
 		goto out;
-	}
 
 	msg = log_from_idx(dumper->cur_idx);
 	l = msg_print_text(msg, 0, syslog, line, size);
@@ -2556,12 +2554,41 @@ bool kmsg_dump_get_line(struct kmsg_dumper *dumper, bool syslog,
 	dumper->cur_idx = log_next(dumper->cur_idx);
 	dumper->cur_seq++;
 	ret = true;
-	raw_spin_unlock_irqrestore(&logbuf_lock, flags);
 out:
 	if (len)
 		*len = l;
 	return ret;
 }
+
+/**
+ * kmsg_dump_get_line - retrieve one kmsg log line
+ * @dumper: registered kmsg dumper
+ * @syslog: include the "<4>" prefixes
+ * @line: buffer to copy the line to
+ * @size: maximum size of the buffer
+ * @len: length of line placed into buffer
+ *
+ * Start at the beginning of the kmsg buffer, with the oldest kmsg
+ * record, and copy one record into the provided buffer.
+ *
+ * Consecutive calls will return the next available record moving
+ * towards the end of the buffer with the youngest messages.
+ *
+ * A return value of FALSE indicates that there are no more records to
+ * read.
+ */
+bool kmsg_dump_get_line(struct kmsg_dumper *dumper, bool syslog,
+			char *line, size_t size, size_t *len)
+{
+	unsigned long flags;
+	bool ret;
+
+	raw_spin_lock_irqsave(&logbuf_lock, flags);
+	ret = kmsg_dump_get_line_nolock(dumper, syslog, line, size, len);
+	raw_spin_unlock_irqrestore(&logbuf_lock, flags);
+
+	return ret;
+}
 EXPORT_SYMBOL_GPL(kmsg_dump_get_line);
 
 /**
@@ -2664,6 +2691,24 @@ out:
 EXPORT_SYMBOL_GPL(kmsg_dump_get_buffer);
 
 /**
+ * kmsg_dump_rewind_nolock - reset the interator (unlocked version)
+ * @dumper: registered kmsg dumper
+ *
+ * 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)
+{
+	dumper->cur_seq = clear_seq;
+	dumper->cur_idx = clear_idx;
+	dumper->next_seq = log_next_seq;
+	dumper->next_idx = log_next_idx;
+}
+
+/**
  * kmsg_dump_rewind - reset the interator
  * @dumper: registered kmsg dumper
  *
@@ -2676,10 +2721,7 @@ void kmsg_dump_rewind(struct kmsg_dumper *dumper)
 	unsigned long flags;
 
 	raw_spin_lock_irqsave(&logbuf_lock, flags);
-	dumper->cur_seq = clear_seq;
-	dumper->cur_idx = clear_idx;
-	dumper->next_seq = log_next_seq;
-	dumper->next_idx = log_next_idx;
+	kmsg_dump_rewind_nolock(dumper);
 	raw_spin_unlock_irqrestore(&logbuf_lock, flags);
 }
 EXPORT_SYMBOL_GPL(kmsg_dump_rewind);
-- 
1.7.10.4

[PATCH 4/4] kdb: Switch to nolock variants of kmsg_dump functions

[Anton Vorontsov]

The locked variants are prone to deadlocks (suppose we got to the
debugger w/ the logbuf lock held), so let's switch to nolock
variants.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 kernel/debug/kdb/kdb_main.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/debug/kdb/kdb_main.c b/kernel/debug/kdb/kdb_main.c
index df17c93..1f91413 100644
--- a/kernel/debug/kdb/kdb_main.c
+++ b/kernel/debug/kdb/kdb_main.c
@@ -2072,8 +2072,8 @@ static int kdb_dmesg(int argc, const char **argv)
 		kdb_set(2, setargs);
 	}
 
-	kmsg_dump_rewind(&dumper);
-	while (kmsg_dump_get_line(&dumper, 1, NULL, 0, NULL))
+	kmsg_dump_rewind_nolock(&dumper);
+	while (kmsg_dump_get_line_nolock(&dumper, 1, NULL, 0, NULL))
 		n++;
 
 	if (lines < 0) {
@@ -2105,8 +2105,8 @@ static int kdb_dmesg(int argc, const char **argv)
 	if (skip >= n || skip < 0)
 		return 0;
 
-	kmsg_dump_rewind(&dumper);
-	while (kmsg_dump_get_line(&dumper, 1, buf, sizeof(buf), &len)) {
+	kmsg_dump_rewind_nolock(&dumper);
+	while (kmsg_dump_get_line_nolock(&dumper, 1, buf, sizeof(buf), &len)) {
 		if (skip) {
 			skip--;
 			continue;
-- 
1.7.10.4

Re: [PATCH for-3.5 1/4] kdb: Revive dmesg command

[Andrew Morton]

On Fri, 20 Jul 2012 17:27:37 -0700
Anton Vorontsov <anton.vorontsov@linaro.org> wrote:

> dmesg command appears to be broken after the printk rework. The old logic
> in kdb code makes no sense in terms of current printk/logging storage
> format, and KDB simply hangs forever.
> 
> This patch revives the command by switching to kmsg_dumper iterator.
> 
> The code is now much more simpler and shorter.
> 
> Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>

This one should me merged into 3.5.1 methinks.  Note that it has been
merged into mainline without a -stable tag.

Re: [PATCH for-3.5 1/4] kdb: Revive dmesg command

[Anton Vorontsov]

On Mon, Jul 23, 2012 at 04:19:27PM -0700, Andrew Morton wrote:
> > dmesg command appears to be broken after the printk rework. The old logic
> > in kdb code makes no sense in terms of current printk/logging storage
> > format, and KDB simply hangs forever.
> > 
> > This patch revives the command by switching to kmsg_dumper iterator.
> > 
> > The code is now much more simpler and shorter.
> > 
> > Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
> 
> This one should me merged into 3.5.1 methinks.  Note that it has been
> merged into mainline without a -stable tag.

Thanks to Linus, it made it into v3.5 release, so I believe there is
no need for -stable.

Cheers,

-- 
Anton Vorontsov
Email: cbouatmailru@gmail.com