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, 5 insertions, 0 deletions

diff --git a/sys/sys/pcpu.h b/sys/sys/pcpu.h
index 00a3cac..fa54bc4 100644
--- a/sys/sys/pcpu.h
+++ b/sys/sys/pcpu.h
@@ -48,6 +48,9 @@
 struct pcb;
 struct thread;
 
+/* Size of the per-cpu console buffer for printf(). */
+#define	PCPU_CONS_BUFR	128
+
 /*
  * This structure maps out the global data that needs to be kept on a
  * per-cpu basis.  The members are accessed via the PCPU_GET/SET/PTR
@@ -71,6 +74,8 @@ struct pcpu {
 	int		pc_ktr_idx;		/* Index into trace table */
 	char		*pc_ktr_buf;
 #endif
+	char		pc_cons_bufr[PCPU_CONS_BUFR];
+						/* Console buffer */
 	PCPU_MD_FIELDS;
 	struct vmmeter	pc_cnt;			/* VM stats counters */
 	struct device	*pc_device;