Re: [Qemu-devel] [PATCH] char: kick main loop after adding a watch

[Richard W.M. Jones]

On Fri, Mar 31, 2017 at 05:43:22PM +0100, Stefan Hajnoczi wrote:
> The ISA serial port device's output can hang when the pipe on stdout
> becomes full.  This is a race condition where the vcpu thread executing
> serial emulation code adds a watch on stdout while the main loop thread
> is blocked in ppoll(2).  If no timer or other event wakes up the main
> loop, there will be no further output from the serial device even when
> the pipe becomes writable.
> 
> Richard W. M. Jones was able to reproduce the hang on recent versions of
> guestfs-tools-c and libglib2 on Fedora 26 hosts.
> 
> This patch kicks the main loop so the next iteration invokes ppoll(2)
> with the watch fd.
> 
> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1435432
> Reported-by: Richard W. M. Jones <address@hidden>
> Tested-by: Richard W. M. Jones <address@hidden>
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
>  chardev/char.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/chardev/char.c b/chardev/char.c
> index 3df1163..6c99c34 100644
> --- a/chardev/char.c
> +++ b/chardev/char.c
> @@ -1059,6 +1059,11 @@ guint qemu_chr_fe_add_watch(CharBackend *be, 
> GIOCondition cond,
>      tag = g_source_attach(src, NULL);
>      g_source_unref(src);
>  
> +    /* The main loop may be in blocked waiting on events in another thread.
> +     * Kick it so the new watch will be added.
> +     */
> +    qemu_notify_event();
> +
>      return tag;
>  }
>  

Obviously Paolo has provided further insights in the other part of
this thread, but from my point of view this does appear to fix the
problem.  I tested it on 3 different physical machines which
previously showed hangs, and on all 3 the test ran for > 30 mins
[before I killed the test] which is far longer than seen before.

Therefore: ACK.

Rich.

-- 
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