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SubjectRe: [PATCH] console: prevent registered consoles from dumping old kernel message over again
On Wed, 12 Jan 2011 14:48:46 +0800
Feng Tang <feng.tang@intel.com> wrote:

> For a platform with many consoles like:
> "console=tty1 console=ttyMFD2 console=ttyS0 earlyprintk=mrst"
>
> Each time when the non "selected_console" (tty1 and ttyMFD2 here) get
> registered, the existing kernel message will be printed out on registered
> consoles again, the "mrst" early console will get some same message for
> 3 times, and "tty1" will get some for twice.
>
> So try to temporarily disable registered console's printing when dump the
> existing kernel messages to the new console.
>
> Signed-off-by: Feng Tang <feng.tang@intel.com>
> ---
> kernel/printk.c | 13 +++++++++++++
> 1 files changed, 13 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/printk.c b/kernel/printk.c
> index f64b899..ba7186b 100644
> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -1245,6 +1245,7 @@ void register_console(struct console *newcon)
> int i;
> unsigned long flags;
> struct console *bcon = NULL;
> + struct console *con;
>
> /*
> * before we register a new CON_BOOT console, make sure we don't
> @@ -1357,8 +1358,20 @@ void register_console(struct console *newcon)
> spin_lock_irqsave(&logbuf_lock, flags);
> con_start = log_start;
> spin_unlock_irqrestore(&logbuf_lock, flags);
> +
> + /*
> + * Disable the existing consoles temporarily to prevent it from
> + * printing out kernel log again.
> + */
> + for_each_console(con)
> + if (con != newcon)
> + con->flags &= ~CON_ENABLED;

Is it racy? If some CPU does an unrelated printk in the middle of all
this, might that get lost? Perhaps not, given the way we pass the
start/end indexes into log_buf[].

> }
> release_console_sem();
> +
> + for_each_console(con)
> + con->flags |= CON_ENABLED;

If a console previously had CON_ENABLED unset, this will bogusly set
it. That will (at least) defeat the intent of a previous
console_stop().

How's about something like this?

--- a/kernel/printk.c~a
+++ a/kernel/printk.c
@@ -113,6 +113,11 @@ static unsigned con_start; /* Index into
static unsigned log_end; /* Index into log_buf: most-recently-written-char + 1 */

/*
+ * If exclusive_console is non-NULL then only this console is to be printed to.
+ */
+static struct console *exclusive_console;
+
+/*
* Array of consoles built from command line options (console=)
*/
struct console_cmdline
@@ -460,6 +465,8 @@ static void __call_console_drivers(unsig
struct console *con;

for_each_console(con) {
+ if (exclusive_console && con != exclusive_console)
+ continue;
if ((con->flags & CON_ENABLED) && con->write &&
(cpu_online(smp_processor_id()) ||
(con->flags & CON_ANYTIME)))
@@ -1358,8 +1365,15 @@ void register_console(struct console *ne
spin_lock_irqsave(&logbuf_lock, flags);
con_start = log_start;
spin_unlock_irqrestore(&logbuf_lock, flags);
+ /*
+ * We're about to replay the log buffer. Only do this to the
+ * just-registered console to avoid excessive message spam to
+ * the already-registered consoles.
+ */
+ exclusive_console = newcon;
}
release_console_sem();
+ exclusive_console = NULL;
console_sysfs_notify();

/*
_


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