linux-kernel - VirtIO console hangs

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Message-ID: <CAGnkfhzjeYU_jmhNkLEayL1FSq5rWkT7T+Nm1ky9QS-vWHUhyw@mail.gmail.com>
Date:   Tue, 28 Aug 2018 12:54:08 +0000
From:   Matteo Croce <mcroce@...hat.com>
To:     Nicholas Piggin <npiggin@...il.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: VirtIO console hangs

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

    Avoid looping with the spinlock held while there is read data
    being returned from the hv driver. Instead note if the entire
    size returned by tty_buffer_request_room was read, and request
    another read poll.

    This limits the critical section lengths, and provides more
    even service to other consoles in case there is a pathological
    condition.

    Signed-off-by: Nicholas Piggin <npiggin@...il.com>
    Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
-- 
Matteo Croce
per aspera ad upstream