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Subject[PATCH 1/4] printk: LOG_CONT and LOG_NEWLINE are separate
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Two log record flags--LOG_CONT and LOG_NEWLINE--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 assertion.

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

So this patch leverages the one-or-the-other nature of these two
flags to simplify a small bit of logic. It makes a some other small
changes to make it obvious that a few things in surrounding (and
related) code are invariant.

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 by
cont_flush() to log_store() either as-is, or modified to include
LOG_NOCONS. There are thus four possible flag combinations supplied
to log_store() by cont_flush(): LOG_CONT; LOG_NEWLINE;
LOG_CONT|LOG_NOCONS; or LOG_NEWLINE|LOG_NOCONS.

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
computed "lflags" value, possibly with LOG_CONT added. The only
possible flag combinations held in "lflags" are: 0; LOG_NEWLINE;
LOG_PREFIX; or LOG_NEWLINE|LOG_PREFIX. And the LOG_CONT flag is
added only when LOG_NEWLINE is not set. So there are six possible
flag combinations supplied by cont_flush(): 0; LOG_NEWLINE;
LOG_PREFIX; LOG_CONT; LOG_PREFIX|LOG_CONT; or LOG_PREFIX|LOG_NEWLINE.

Therefore log_store() is never provided (so never records) a flag
value that contains both LOG_CONT and LOG_NEWLINE.

Meanwhile, the "cont" flags field is only ever assigned value 0, or
a value passed to cont_flush(). As mentioned above, cont_flush() is
never provided more than one flag value. The only values assigned
to "syslog_prev" and "console_prev" are 0 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 most one of LOG_CONT and LOG_NEWLINE will ever
be set in a log record flags field. And as a result, we know that
LOG_CONT being set implies LOG_NEWLINE is not set (and vice versa).

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.
- Explicitly pass NULL rather than "dict" (and 0 as its length)
to log_store() in one spot in vprintk_emit(), because we know
the value of dict at that point is always NULL.

Signed-off-by: Alex Elder <elder@linaro.org>
---
kernel/printk/printk.c | 12 +++++-------
1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 13e839d..301ade3 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;
}

do {
@@ -1642,7 +1640,7 @@ asmlinkage int vprintk_emit(int facility, int level,
/* mark and strip a trailing newline */
if (text_len && text[text_len-1] == '\n') {
text_len--;
- lflags |= LOG_NEWLINE;
+ lflags = LOG_NEWLINE;
}

/* strip kernel syslog prefix and extract log level or control flags */
@@ -1672,7 +1670,7 @@ asmlinkage int vprintk_emit(int facility, int level,
level = default_message_loglevel;

if (dict)
- lflags |= LOG_PREFIX|LOG_NEWLINE;
+ lflags = LOG_PREFIX|LOG_NEWLINE;

if (!(lflags & LOG_NEWLINE)) {
/*
@@ -1688,7 +1686,7 @@ asmlinkage int vprintk_emit(int facility, int level,
else
printed_len += log_store(facility, level,
lflags | LOG_CONT, 0,
- dict, dictlen, text, text_len);
+ NULL, 0, text, text_len);
} else {
bool stored = false;

--
1.9.1


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