Add a cnputs() function to write a string to the console with

a lock to prevent interspersed strings written from different CPUs
at the same time.

To avoid putting a buffer on the stack or having to malloc one,
space is incorporated in the per-cpu structure. The buffer
size if 128 bytes; chosen because it's the next power of 2 size
up from 80 characters.

String writes to the console are buffered up the end of the line
or until the buffer fills. Then the buffer is flushed to all
console devices.

Existing low level console output via cnputc() is unaffected by
this change. ithread calls to log() are also unaffected to avoid
blocking those threads.

A minor change to the behaviour in a panic situation is that
console output will still be buffered, but won't be written to
a tty as before. This should prevent interspersed panic output
as a number of CPUs panic before we end up single threaded
running ddb.

Reviewed by:	scottl, jhb
MFC after:	2 weeks

1 files changed, 25 insertions, 0 deletions

diff --git a/sys/kern/tty_cons.c b/sys/kern/tty_cons.c
index f65f7ad..c0f8692 100644
--- a/sys/kern/tty_cons.c
+++ b/sys/kern/tty_cons.c
@@ -41,6 +41,8 @@ __FBSDID("$FreeBSD$");
 
 #include <sys/param.h>
 #include <sys/systm.h>
+#include <sys/lock.h>
+#include <sys/mutex.h>
 #include <sys/conf.h>
 #include <sys/cons.h>
 #include <sys/fcntl.h>
@@ -117,6 +119,8 @@ static u_char console_pausing;		/* pause after each line during probe */
 static char *console_pausestr=
 "<pause; press any key to proceed to next line or '.' to end pause mode>";
 struct tty *constty;			/* pointer to console "window" tty */
+static struct mtx cnputs_mtx;		/* Mutex for cnputs(). */
+static int use_cnputs_mtx = 0;		/* != 0 if cnputs_mtx locking reqd. */
 
 static void constty_timeout(void *arg);
 
@@ -639,6 +643,24 @@ cnputc(int c)
 	}
 }
 
+void
+cnputs(char *p)
+{
+	int c;
+	int unlock_reqd = 0;
+
+	if (use_cnputs_mtx) {
+		mtx_lock_spin(&cnputs_mtx);
+		unlock_reqd = 1;
+	}
+
+	while ((c = *p++) != '\0')
+		cnputc(c);
+
+	if (unlock_reqd)
+		mtx_unlock_spin(&cnputs_mtx);
+}
+
 static int consmsgbuf_size = 8192;
 SYSCTL_INT(_kern, OID_AUTO, consmsgbuf_size, CTLFLAG_RW, &consmsgbuf_size, 0,
     "");
@@ -708,6 +730,9 @@ cn_drvinit(void *unused)
 {
 
 	make_dev(&cn_cdevsw, 0, UID_ROOT, GID_WHEEL, 0600, "console");
+
+	mtx_init(&cnputs_mtx, "cnputs_mtx", NULL, MTX_SPIN | MTX_NOWITNESS);
+	use_cnputs_mtx = 1;
 }
 
 SYSINIT(cndev, SI_SUB_DRIVERS, SI_ORDER_MIDDLE, cn_drvinit, NULL)