[PATCH v7 4/7] printk: LOG_CONT and LOG_NEWLINE are opposites

From: Alex Elder
Date: Mon Jul 28 2014 - 09:30:33 EST

Two log record flags--LOG_CONT and LOG_NEWLINE--are mutually
exclusive. That is, one or the other is always set, but they are
never both set at the same time in a log record flags field. What
follows is a great deal of explanation that aims to prove this

Having that knowledge allows us to simplify a bit of logic, and with
a little more work (in follow-on patches) it allows us to do without
a flag value, simplifying things.

Log record flags are held in the "cont" continuation buffer, as well
as in "syslog_prev" and "console_prev". Those are discussed later.

Other than that, log record flags are only set in log_store():
msg->flags = flags & 0x1f;
There are 5 places log_store() is called: twice from cont_flush();
and three times from vprintk_emit().

Only two single-flag values are ever passed to cont_flush():
LOG_CONT; and LOG_NEWLINE. That passed-in value is provided to
log_store() either as-is, or modified to include LOG_NOCONS. There
are thus four possible flag combinations supplied to log_store() by

The first call vprintk_emit() makes to log_store() passes a flags
value of LOG_PREFIX|LOG_NEWLINE. The second and third calls pass a
locally-computed "lflags" value, possibly with LOG_CONT added. The
only possible flag combinations held in "lflags" are: 0;
is not set, LOG_CONT flag is added in the call to log_store(). So
there are four possible flag combinations supplied by vprintk_emit():

Therefore log_store() is never provided (so never records) a flag
value that contains both LOG_CONT and LOG_NEWLINE, and one of those
flags is always present.

Meanwhile, the "cont" flags field is only ever assigned value 0, or
a value passed to cont_flush(). The value of cont.flags is only
ever *used* if cont.flushed is true, and that only gets set after
the cont.flags field is assigned to the value passed to
cont_flush(). As mentioned above, cont_flush() is never provided
more than one flag value, and it's always either LOG_NEWLINE
or LOG_CONT. Therefore, for all intents and purposes, cont.flags
only ever holds LOG_NEWLINE or LOG_CONT.

The only values assigned to "syslog_prev" and "console_prev" are the
initial value LOG_NEWLINE or the flags value from a log record. So
none of these ever hold LOG_CONT and LOG_NEWLINE at the same time.

This proves that at LOG_CONT and LOG_NEWLINE are in fact mutually
exclusive flags.

This patch makes the following changes:
- Changes two spots in msg_print_txt() so they no longer bother
checking for LOG_NEWLINE once its known that LOG_CONT is set.
- Changes two |= assignments in vprintk_emit() to be simple
assignments instead, because the result is known to be the
same and it makes it obvious no other flags are involved.
- Assigns LOG_CONT to "lflags" in vprintk_emit() if it does not
contain LOG_NEWLINE (rather than just adding it at the time
of the call to log_store()).
- Adds a short explanatory comment.

Signed-off-by: Alex Elder <elder@xxxxxxxxxx>
Reviewed-by: Petr MlÃdek <pmladek@xxxxxxx>
kernel/printk/printk.c | 21 ++++++++++++---------
1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 5c32938..4b063c8 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1006,11 +1006,9 @@ static size_t msg_print_text(const struct printk_log *msg, enum log_flags prev,
prefix = false;

if (msg->flags & LOG_CONT) {
- if ((prev & LOG_CONT) && !(prev & LOG_NEWLINE))
+ if (prev & LOG_CONT)
prefix = false;
- if (!(msg->flags & LOG_NEWLINE))
- newline = false;
+ newline = false;

if ((prev & LOG_CONT) && (msg->flags & LOG_PREFIX) && len < size) {
@@ -1657,10 +1655,16 @@ asmlinkage int vprintk_emit(int facility, int level,
text_len += vscnprintf(text + text_len,
sizeof(textbuf) - text_len, fmt, args);

- /* mark and strip a trailing newline */
+ /*
+ * If there's a trailing newline, flag it and strip it off.
+ * Otherwise we assume this is a partial log message, to be
+ * continued with the next call.
+ */
if (text_len && text[text_len-1] == '\n') {
- lflags |= LOG_NEWLINE;
+ lflags = LOG_NEWLINE;
+ } else {
+ lflags = LOG_CONT;

/* strip kernel syslog prefix and extract log level or control flags */
@@ -1690,7 +1694,7 @@ asmlinkage int vprintk_emit(int facility, int level,
level = default_message_loglevel;

if (dict)

if (!(lflags & LOG_NEWLINE)) {
@@ -1704,8 +1708,7 @@ asmlinkage int vprintk_emit(int facility, int level,
if (cont_add(facility, level, text, text_len))
printed_len += text_len;
- printed_len += log_store(facility, level,
- lflags | LOG_CONT, 0,
+ printed_len += log_store(facility, level, lflags, 0,
dict, dictlen, text, text_len);
} else {
bool stored = false;

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