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Commit 3feebbb5 authored by Hendrik Brueckner's avatar Hendrik Brueckner Committed by Benjamin Herrenschmidt
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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: default avatarChristian Borntraeger <borntraeger@de.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com> (virtio_console)
Signed-off-by: default avatarHendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: default avatarBenjamin Herrenschmidt <benh@kernel.crashing.org>
parent 56ce9e2b

drivers/char/hvc_console.c

+11 −9
Original line number Diff line number Diff line
@@ -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)