Re: [Qemu-devel] [PULL 3/5] serial: change retry logic to avoid concurrency

[Pavel Hrdina <phrdina@redhat.com>]

On 14.7.2014 17:49, Paolo Bonzini wrote:
> From: Kirill Batuzov <batuzovk@ispras.ru>
> 
> Whenever serial_xmit fails to transmit a byte it adds a watch that would
> call it again when the "line" becomes ready. This results in a retry
> chain:
>   serial_xmit -> add_watch -> serial_xmit
> Each chain is able to transmit one character, and for every character
> passed to serial by the guest driver a new chain is spawned.
> 
> The problem lays with the fact that a new chain is spawned even when
> there is one already waiting on the watch. So there can be several retry
> chains waiting concurrently on one "line". Every chain tries to transmit
> current character, so character order is not messed up. But also every
> chain increases retry counter (tsr_retry). If there are enough
> concurrent chains this counter will hit MAX_XMIT_RETRY value and
> the character will be dropped.
> 
> To reproduce this bug you need to feed serial output to some program
> consuming it slowly enough. A python script from bug #1335444
> description is an example of such program.
> 
> This commit changes retry logic in the following way to avoid
> concurrency: instead of spawning a new chain for each character being
> transmitted spawn only one and make it transmit characters until FIFO is
> empty.
> 
> The change consists of two parts:
>  - add a do {} while () loop in serial_xmit (diff is a bit erratic
>    for this part, diff -w will show actual change),
>  - do not call serial_xmit from serial_ioport_write if there is one
>    waiting on the watch already.
> 
> This should fix another issue causing bug #1335444.
> 
> Signed-off-by: Kirill Batuzov <batuzovk@ispras.ru>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>


Hi, this commit introduced a regression with serial console. The issue
is that if you start a guest with serial console:

-chardev pty,id=charserial0 -device
isa-serial,chardev=charserial0,id=serial0

the guest hang during boot for a long time and I'm not even sure it
it ever boot up. The last message printed out by kernel is:

"[   0.000000] console [tty0] enabled"

If you connect to the serial console than the guest continue
booting immediately.

Pavel