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SubjectRe: Regression from patch 'tty: hvc: hvc_poll() break hv read loop'
On Tue, 4 Sep 2018 15:16:35 -0600
Jason Gunthorpe <jgg@mellanox.com> wrote:

> On Wed, Sep 05, 2018 at 07:15:29AM +1000, Nicholas Piggin wrote:
> > On Tue, 4 Sep 2018 11:48:08 -0600
> > Jason Gunthorpe <jgg@mellanox.com> wrote:
> >
> > > Hi Nicholas,
> > >
> > > I am testing 4.19-rc2 and I see bad behavior with my qemu hvc0
> > > console..
> > >
> > > Running interactive with qemu (qemu-2.11.2-1.fc28) on the console
> > > providing hvc0, using options like:
> > >
> > > -nographic
> > > -chardev stdio,id=stdio,mux=on,signal=off
> > > -mon chardev=stdio
> > > -device isa-serial,chardev=stdio
> > > -device virtio-serial-pci
> > > -device virtconsole,chardev=stdio
> > >
> > > I see the hvc0 console hang regularly, ie doing something like 'up
> > > arrow' in bash causes the hvc0 console to hang. Prior kernels worked
> > > OK.
> > >
> > > Any ideas? I'm not familiar with this code.. Thanks!
> >
> > Yes I have had another report, I'm working on a fix. Sorry it has taken
> > a while and thank you for the report.
>
> Okay, let me know when you have a fix and I will be able to test it
> for you!

Can you try this?

diff --git a/drivers/tty/hvc/hvc_console.c b/drivers/tty/hvc/hvc_console.c
index 5414c4a87bea..f5fc3ba49130 100644
--- a/drivers/tty/hvc/hvc_console.c
+++ b/drivers/tty/hvc/hvc_console.c
@@ -49,6 +49,8 @@
#define N_OUTBUF 16
#define N_INBUF 16

+#define HVC_ATOMIC_READ_MAX 128
+
#define __ALIGNED__ __attribute__((__aligned__(sizeof(long))))

static struct tty_driver *hvc_driver;
@@ -522,6 +524,8 @@ static int hvc_write(struct tty_struct *tty, const unsigned char *buf, int count
return -EIO;

while (count > 0) {
+ int ret;
+
spin_lock_irqsave(&hp->lock, flags);

rsize = hp->outbuf_size - hp->n_outbuf;
@@ -537,10 +541,13 @@ static int hvc_write(struct tty_struct *tty, const unsigned char *buf, int count
}

if (hp->n_outbuf > 0)
- hvc_push(hp);
+ ret = hvc_push(hp);

spin_unlock_irqrestore(&hp->lock, flags);

+ if (!ret)
+ break;
+
if (count) {
if (hp->n_outbuf > 0)
hvc_flush(hp);
@@ -669,8 +676,8 @@ static int __hvc_poll(struct hvc_struct *hp, bool may_sleep)
if (!hp->irq_requested)
poll_mask |= HVC_POLL_READ;

+ read_again:
/* Read data if any */
-
count = tty_buffer_request_room(&hp->port, N_INBUF);

/* If flip is full, just reschedule a later read */
@@ -717,9 +724,23 @@ 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;
+ read_total += n;
+
+ if (may_sleep) {
+ /* Keep going until the flip is full */
+ spin_unlock_irqrestore(&hp->lock, flags);
+ cond_resched();
+ spin_lock_irqsave(&hp->lock, flags);
+ goto read_again;
+ } else if (read_total < HVC_ATOMIC_READ_MAX) {
+ /* Break and defer if it's a large read in atomic */
+ goto read_again;
+ }
+
+ /*
+ * Latency break, schedule another poll immediately.
+ */
+ poll_mask |= HVC_POLL_READ;

out:
/* Wakeup write queue if necessary */
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