Slightly improve the design of the TTY buffer.

The TTY buffers used the standard <sys/queue.h> lists. Unfortunately
they have a big shortcoming. If you want to have a double linked list,
but no tail pointer, it's still not possible to obtain the previous
element in the list. Inside the buffers we don't need them. This is why
I switched to custom linked list macros. The macros will also keep track
of the amount of items in the list. Because it doesn't use a sentinel,
we can just initialize the queues with zero.

In its simplest form (the output queue), we will only keep two
references to blocks in the queue, namely the head of the list and the
last block in use. All free blocks are stored behind the last block in
use.

I noticed there was a very subtle bug in the previous code: in a very
uncommon corner case, it would uma_zfree() a block in the queue before
calling memcpy() to extract the data from the block.

1 files changed, 0 insertions, 1 deletions

diff --git a/sys/dev/snp/snp.c b/sys/dev/snp/snp.c
index 8d6485c..4a48037 100644
--- a/sys/dev/snp/snp.c
+++ b/sys/dev/snp/snp.c
@@ -127,7 +127,6 @@ snp_open(struct cdev *dev, int flag, int mode, struct thread *td)
 
 	/* Allocate per-snoop data. */
 	ss = malloc(sizeof(struct snp_softc), M_SNP, M_WAITOK|M_ZERO);
-	ttyoutq_init(&ss->snp_outq);
 	cv_init(&ss->snp_outwait, "snp out");
 
 	devfs_set_cdevpriv(ss, snp_dtor);