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Date:   Fri, 31 Aug 2018 13:49:43 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Matteo Croce <mcroce@...hat.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        linux-kernel@...r.kernel.org
Subject: Re: VirtIO console hangs

On Tue, 28 Aug 2018 15:00:14 +0000
Matteo Croce <mcroce@...hat.com> wrote:

> On Tue, Aug 28, 2018 at 2:35 PM Nicholas Piggin <npiggin@...il.com> wrote:
> >
> > On Tue, 28 Aug 2018 12:54:08 +0000
> > Matteo Croce <mcroce@...hat.com> wrote:
> >  
> > > With kernel 4.19.0-rc1 virtio_console hangs very often.
> > > I can always trigger the bug by pasting some characters in the
> > > terminal window, the console will stop receiving keypresses, but I can
> > > still see output from the console.
> > > Stangely, logging in the VM via SSH and sending lot of data to hvc0,
> > > like 'dmesg >/dev/hvc0' will fix the issue until the next paste.
> > >
> > > I did a git bisect and I've found that this is the offending commit,
> > > reverting it fixes it.
> > >
> > > Cheers,
> > >
> > > commit ec97eaad1383ab2500fcf9a07ade6044fbcc67f5
> > > Author: Nicholas Piggin <npiggin@...il.com>
> > > Date:   Tue May 1 00:55:54 2018 +1000
> > >
> > >     tty: hvc: hvc_poll() break hv read loop  
> >
> > Thanks for the report. I can't immediately see what the problem
> > is. Can you try get a stack trace of where it is stuck?
> >  
> 
> I tried but didn't get one.
> 
> > Perhaps try this patch if you have time (it's a bit of a shot
> > in the dark).
> >  
> 
> Yes it seems to fix. Thanks!

Okay sorry for the delay, I can reproduce it here and found a better
fix, if I could trouble you to test again.

[PATCH] tty: hvc: hvc_poll() fix read loop hang

Patch ec97eaad1383 ("tty: hvc: hvc_poll() break hv read loop") causes
the virtio console to hang if you paste a bunch of characters to it.

The reason is that get_chars must return 0 before we can be sure the
driver will kick or poll input again, but this patch only scheduled a
poll if get_chars had returned a full count. Change this to poll on
> 0 count.

Fixes: ec97eaad1383 ("tty: hvc: hvc_poll() break hv read loop")
Reported-by: Matteo Croce <mcroce@...hat.com>
Signed-off-by: Nicholas Piggin <npiggin@...il.com>
---
 drivers/tty/hvc/hvc_console.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/drivers/tty/hvc/hvc_console.c
b/drivers/tty/hvc/hvc_console.c index 5414c4a87bea..219369aebb1f 100644
--- a/drivers/tty/hvc/hvc_console.c
+++ b/drivers/tty/hvc/hvc_console.c
@@ -717,10 +717,13 @@ static int __hvc_poll(struct hvc_struct *hp, bool
may_sleep) #endif /* CONFIG_MAGIC_SYSRQ */
 		tty_insert_flip_char(&hp->port, buf[i], 0);
 	}
-	if (n == count)
-		poll_mask |= HVC_POLL_READ;
 	read_total = n;
 
+	/* Activity is occurring, so reset the polling backoff value to
+	   a minimum for performance. */
+	timeout = MIN_TIMEOUT;
+	poll_mask |= HVC_POLL_READ;
+
  out:
 	/* Wakeup write queue if necessary */
 	if (hp->do_wakeup) {
@@ -730,13 +733,8 @@ static int __hvc_poll(struct hvc_struct *hp, bool
may_sleep) bail:
 	spin_unlock_irqrestore(&hp->lock, flags);
 
-	if (read_total) {
-		/* Activity is occurring, so reset the polling backoff
value to
-		   a minimum for performance. */
-		timeout = MIN_TIMEOUT;
-
+	if (read_total)
 		tty_flip_buffer_push(&hp->port);
-	}
 	tty_kref_put(tty);
 
 	return poll_mask;
-- 
2.18.0

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