Turn console printf buffering into a kernel option and only on

by default for sun4v where it is absolutely required.

This change moves the buffer from struct pcpu to the stack to avoid
using the critical section which created a LOR in a couple of cases
due to interaction with the tty code and kqueue. The LOR can't be
fixed with the critical section and the pcpu buffer can't be used
without the critical section.

Putting the buffer on the stack was my initial solution, but it was
pointed out that the stress on the stack might cause problems
depending on the call path. We don't have a way of creating tests
for those possible cases, so it's best to leave this as an option
for the time being. In time we may get enough data to enable this
option more generally.

1 files changed, 23 insertions, 14 deletions

diff --git a/sys/kern/subr_prf.c b/sys/kern/subr_prf.c
index c53bf98..d7c87a9 100644
--- a/sys/kern/subr_prf.c
+++ b/sys/kern/subr_prf.c
@@ -38,6 +38,7 @@
 __FBSDID("$FreeBSD$");
 
 #include "opt_ddb.h"
+#include "opt_printf.h"
 
 #include <sys/param.h>
 #include <sys/systm.h>
@@ -291,18 +292,24 @@ printf(const char *fmt, ...)
 	va_list ap;
 	struct putchar_arg pca;
 	int retval;
-
-	critical_enter();
+#ifdef PRINTF_BUFR_SIZE
+	char bufr[PRINTF_BUFR_SIZE];
+#endif
 
 	va_start(ap, fmt);
 	pca.tty = NULL;
 	pca.flags = TOCONS | TOLOG;
 	pca.pri = -1;
-	pca.p_bufr = (char *) PCPU_PTR(cons_bufr);
+#ifdef PRINTF_BUFR_SIZE
+	pca.p_bufr = bufr;
 	pca.p_next = pca.p_bufr;
-	pca.n_bufr = PCPU_CONS_BUFR;
-	pca.remain = PCPU_CONS_BUFR;
+	pca.n_bufr = sizeof(bufr);
+	pca.remain = sizeof(bufr);
 	*pca.p_next = '\0';
+#else
+	/* Don't buffer console output. */
+	pca.p_bufr = NULL;
+#endif
 
 	retval = kvprintf(fmt, putchar, &pca, 10, ap);
 	va_end(ap);
@@ -314,8 +321,6 @@ printf(const char *fmt, ...)
 	if (!panicstr)
 		msgbuftrigger = 1;
 
-	critical_exit();
-
 	return (retval);
 }
 
@@ -324,17 +329,23 @@ vprintf(const char *fmt, va_list ap)
 {
 	struct putchar_arg pca;
 	int retval;
-
-	critical_enter();
+#ifdef PRINTF_BUFR_SIZE
+	char bufr[PRINTF_BUFR_SIZE];
+#endif
 
 	pca.tty = NULL;
 	pca.flags = TOCONS | TOLOG;
 	pca.pri = -1;
-	pca.p_bufr = (char *) PCPU_PTR(cons_bufr);
+#ifdef PRINTF_BUFR_SIZE
+	pca.p_bufr = bufr;
 	pca.p_next = pca.p_bufr;
-	pca.n_bufr = PCPU_CONS_BUFR;
-	pca.remain = PCPU_CONS_BUFR;
+	pca.n_bufr = sizeof(bufr);
+	pca.remain = sizeof(bufr);
 	*pca.p_next = '\0';
+#else
+	/* Don't buffer console output. */
+	pca.p_bufr = NULL;
+#endif
 
 	retval = kvprintf(fmt, putchar, &pca, 10, ap);
 
@@ -345,8 +356,6 @@ vprintf(const char *fmt, va_list ap)
 	if (!panicstr)
 		msgbuftrigger = 1;
 
-	critical_exit();
-
 	return (retval);
 }