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Message-Id: <1406037717-9670-3-git-send-email-elder@linaro.org>
Date: Tue, 22 Jul 2014 09:01:57 -0500
From: Alex Elder <elder@...aro.org>
To: akpm@...ux-foundation.org
Cc: kay@...y.org, pmladek@...e.cz, bp@...e.de, john.stultz@...aro.org,
jack@...e.cz, linux-kernel@...r.kernel.org
Subject: [PATCH v2 2/2] printk: improve some commentary; tidy up
Add some comments to explain how the log flags are used to control
how records get formatted. Also add and refine some comments in
vprintk_emit().
Now that we're done fixing up log record flags, simplify how they're
used in computing some local variable values in msg_print_text().
Use a local variable "flush_cont" in vprintk_emit() to factor out a
common expression used later in that function.
Signed-off-by: Alex Elder <elder@...aro.org>
---
kernel/printk/printk.c | 63 ++++++++++++++++++++++++++++++++------------------
1 file changed, 41 insertions(+), 22 deletions(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 49c9238..cecdc1b 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -141,6 +141,12 @@ EXPORT_SYMBOL(console_set_on_cmdline);
static int console_may_schedule;
/*
+ * Each call to printk() fills a record in a circular log buffer.
+ * The contents of the log buffer are read by various subsystems
+ * (including the console subsystem), each of which formats the
+ * content of log buffers for human consumption. Flags in each
+ * log record are used to track formatting-related state.
+ *
* The printk log buffer consists of a chain of concatenated variable
* length records. Every record starts with a record header, containing
* the overall length of the record.
@@ -150,7 +156,7 @@ static int console_may_schedule;
* are stored..
*
* If the heads indicate available messages, the length in the header
- * tells the start next message. A length == 0 for the next message
+ * tells the start of the next message. A length == 0 for the next message
* indicates a wrap-around to the beginning of the buffer.
*
* Every record carries the monotonic timestamp in microseconds, as well as
@@ -192,6 +198,16 @@ static int console_may_schedule;
* 67 "g"
* 0032 00 00 00 padding to next message header
*
+ * If a printk() call contains no newline, its content is saved in a
+ * special "cont" buffer rather than being written directly into the
+ * log. One or more follow-in printk() calls from the same source
+ * can then be combined into a single newline-terminated message (if
+ * possible) before the combined result is saved into a log record.
+ * Occasionally a buffered/partial message needs to be flushed to
+ * the log before the logically next printk() call is seen. When
+ * this occurs, the incomplete record (with no LOG_NEWLINE) will
+ * be followed by a new record marked LOG_PREFIX.
+ *
* The 'struct printk_log' buffer header must never be directly exported to
* userspace, it is a kernel-private implementation detail that might
* need to be changed in the future, when the requirements change.
@@ -1005,17 +1021,14 @@ static size_t msg_print_text(const struct printk_log *msg, enum log_flags prev,
{
const char *text = log_text(msg);
size_t text_size = msg->text_len;
- bool prefix = true;
- bool newline = true;
size_t len = 0;
+ bool prefix;
+ bool newline;
- if (!(prev & LOG_NEWLINE) && !(msg->flags & LOG_PREFIX))
- prefix = false;
-
- if (!(msg->flags & LOG_NEWLINE))
- newline = false;
-
- if (!(prev & LOG_NEWLINE) && (msg->flags & LOG_PREFIX) && len < size) {
+ prefix = (prev & LOG_NEWLINE) || (msg->flags & LOG_PREFIX);
+ newline = !!(msg->flags & LOG_NEWLINE);
+ /* Insert a newline if we're terminating the previous line early */
+ if (prefix && !(prev & LOG_NEWLINE) && len < size) {
if (buf)
buf[len++] = '\n';
else
@@ -1600,6 +1613,7 @@ asmlinkage int vprintk_emit(int facility, int level,
int this_cpu;
int printed_len = 0;
bool in_sched = false;
+ bool flush_cont = false;
/* cpu currently holding logbuf_lock in this function */
static volatile unsigned int logbuf_cpu = UINT_MAX;
@@ -1698,12 +1712,16 @@ asmlinkage int vprintk_emit(int facility, int level,
if (dict)
lflags = LOG_PREFIX|LOG_NEWLINE;
+ /*
+ * If the previous printk() call had no newline, it will be buffered.
+ * If the buffered message was produced by someone else, or if this
+ * call is forcing a new record, we will need to flush the buffer
+ * rather than merge this message into it.
+ */
+ flush_cont = (cont.owner != current) || (lflags & LOG_PREFIX);
if (!(lflags & LOG_NEWLINE)) {
- /*
- * Flush the conflicting buffer. An earlier newline was missing,
- * or another task also prints continuation lines.
- */
- if (cont.len && (lflags & LOG_PREFIX || cont.owner != current))
+ /* If the buffered record conflicts, flush it first. */
+ if (cont.len && flush_cont)
cont_flush(LOG_NEWLINE);
/* buffer line if possible, otherwise store it right away */
@@ -1716,20 +1734,21 @@ asmlinkage int vprintk_emit(int facility, int level,
bool stored = false;
/*
- * If an earlier newline was missing and it was the same task,
- * either merge it with the current buffer and flush, or if
- * there was a race with interrupts (prefix == true) then just
- * flush it out and store this line separately.
- * If the preceding printk was from a different task and missed
- * a newline, flush and append the newline.
+ * If there's a buffered message, try to merge with
+ * it, then flush whatever's buffered to the log.
*/
if (cont.len) {
- if (cont.owner == current && !(lflags & LOG_PREFIX))
+ if (!flush_cont)
stored = cont_add(facility, level, text,
text_len);
cont_flush(LOG_NEWLINE);
}
+ /*
+ * Record how much we just formatted. If cont_add() didn't
+ * combine this message with the buffered one we still have
+ * to store this one to the log.
+ */
if (stored)
printed_len += text_len;
else
--
1.9.1
--
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