hvc_console: Fix loop if put_char() returns 0

If put_char() routine of a hvc console backend returns 0, then the
hvc console starts looping in the following scenarios:

1. hvc_console_print()
	If put_char() returns 0 then the while loop may loop forever.
	I have added the missing check for 0 to throw away console messages.

2. khvcd may loop:
	The thread calls hvc_poll() --> hvc_push()... if there are still
	buffered data then the HVC_POLL_WRITE bit is set and causes the
	khvcd thread to loop (if yield() returns immediately).

	However, instead of looping, the khvcd thread could sleep for
	MIN_TIMEOUT (doing the same as for get_chars()).
	The MIN_TIMEOUT is set if hvc_push() was not able to write
	data to the backend. If data has been written, the timeout is
	set to 0 to immediately re-schedule hvc_poll().

Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com> (virtio_console)
Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 11 insertions, 9 deletions

diff --git a/drivers/char/hvc_console.c b/drivers/char/hvc_console.c
index c9d16665c960..3650c904401e 100644
--- a/drivers/char/hvc_console.c
+++ b/drivers/char/hvc_console.c
@@ -161,7 +161,7 @@ static void hvc_console_print(struct console *co, const char *b,
                         }
                 } else {
                         r = cons_ops[index]->put_chars(vtermnos[index], c, i);
-                        if (r < 0) {
+                        if (r <= 0) {
                                 /* throw away chars on error */
                                 i = 0;
                         } else if (r > 0) {
@@ -431,7 +431,7 @@ static void hvc_hangup(struct tty_struct *tty)
  * Push buffered characters whether they were just recently buffered or waiting
  * on a blocked hypervisor.  Call this function with hp->lock held.
  */
-static void hvc_push(struct hvc_struct *hp)
+static int hvc_push(struct hvc_struct *hp)
 {
         int n;
 
@@ -439,7 +439,7 @@ static void hvc_push(struct hvc_struct *hp)
         if (n <= 0) {
                 if (n == 0) {
                         hp->do_wakeup = 1;
-                        return;
+                        return 0;
                 }
                 /* throw away output on error; this happens when
                    there is no session connected to the vterm. */
@@ -450,6 +450,8 @@ static void hvc_push(struct hvc_struct *hp)
                 memmove(hp->outbuf, hp->outbuf + n, hp->n_outbuf);
         else
                 hp->do_wakeup = 1;
+
+        return n;
 }
 
 static int hvc_write(struct tty_struct *tty, const unsigned char *buf, int count)
@@ -538,16 +540,20 @@ int hvc_poll(struct hvc_struct *hp)
         char buf[N_INBUF] __ALIGNED__;
         unsigned long flags;
         int read_total = 0;
+        int written_total = 0;
 
         spin_lock_irqsave(&hp->lock, flags);
 
         /* Push pending writes */
         if (hp->n_outbuf > 0)
-                hvc_push(hp);
+                written_total = hvc_push(hp);
 
         /* Reschedule us if still some write pending */
-        if (hp->n_outbuf > 0)
+        if (hp->n_outbuf > 0) {
                 poll_mask |= HVC_POLL_WRITE;
+                /* If hvc_push() was not able to write, sleep a few msecs */
+                timeout = (written_total) ? 0 : MIN_TIMEOUT;
+        }
 
         /* No tty attached, just skip */
         tty = hp->tty;
@@ -659,10 +665,6 @@ static int khvcd(void *unused)
                         poll_mask |= HVC_POLL_READ;
                 if (hvc_kicked)
                         continue;
-                if (poll_mask & HVC_POLL_WRITE) {
-                        yield();
-                        continue;
-                }
                 set_current_state(TASK_INTERRUPTIBLE);
                 if (!hvc_kicked) {
                         if (poll_mask == 0)