linux-kernel - [PATCH 1/3] tty: hvc: hvc

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Message-Id: <20180905121439.23809-2-npiggin@gmail.com>
Date:   Wed,  5 Sep 2018 22:14:37 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Nicholas Piggin <npiggin@...il.com>, Jiri Slaby <jslaby@...e.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Matteo Croce <mcroce@...hat.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Leon Romanovsky <leonro@...lanox.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org (op)
Subject: [PATCH 1/3] tty: hvc: hvc_poll() fix read loop hang

Patch ec97eaad1383 ("tty: hvc: hvc_poll() break hv read loop") causes
the virtio console to hang at times (e.g., 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 change only scheduled a
poll if get_chars had returned a full count. Change this to poll on
any > 0 count.

Fixes: ec97eaad1383 ("tty: hvc: hvc_poll() break hv read loop")
Reported-by: Matteo Croce <mcroce@...hat.com>
Reported-by: Jason Gunthorpe <jgg@...lanox.com>
Tested-by: Matteo Croce <mcroce@...hat.com>
Tested-by: Leon Romanovsky <leonro@...lanox.com>
Signed-off-by: Nicholas Piggin <npiggin@...il.com>
---
 drivers/tty/hvc/hvc_console.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/hvc/hvc_console.c b/drivers/tty/hvc/hvc_console.c
index 5414c4a87bea..c917749708d2 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;
 
+	/*
+	 * Latency break, schedule another poll immediately.
+	 */
+	poll_mask |= HVC_POLL_READ;
+
  out:
 	/* Wakeup write queue if necessary */
 	if (hp->do_wakeup) {
-- 
2.18.0