Re: [PATCH 1/1] tests/qtest/ahci: fix a racy deadlock in the engine stop test

[Fabiano Rosas <farosas@suse.de>]

"Denis V. Lunev" <den@openvz.org> writes:

> From: Denis V. Lunev <den@openvz.org>
>
> test_write_engine_stop_in_flight() resumes the suspended backend write
> and goes straight on to restart the command engine and read the second
> sector back. Whether the write has retired by then is a race. While it
> is still outstanding the drive is busy, and handle_cmd() drops a
> command that arrives in that state instead of queueing it. Nothing
> retries the drop, so the PxCI slot stays set and no D2H FIS is posted.
> ahci_command_wait() polls both with no timeout, so the test deadlocks
> until the harness kills it.
>
> Drain the port before issuing the read, so the abandoned write has
> retired and the engine is idle. Host load stretches the gap between the
> resume and the write completing, which is why the deadlock only shows
> up on a loaded machine.
>
> Fixes: d05ae87e7a6a ("tests/qtest/ahci: regression test for a PIO write vs. engine stop")
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> ---
>  tests/qtest/ahci-test.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/tests/qtest/ahci-test.c b/tests/qtest/ahci-test.c
> index b143862ce7..2552fa39f1 100644
> --- a/tests/qtest/ahci-test.c
> +++ b/tests/qtest/ahci-test.c
> @@ -1914,6 +1914,12 @@ static void test_write_engine_stop_in_flight(void)
>  
>      g_free(qtest_hmp(ahci->parent->qts, "qemu-io drive0 \"resume wr\""));
>  
> +    /*
> +     * Retire the abandoned write. handle_cmd() drops a command that arrives
> +     * while the drive is still busy and nothing retries it.
> +     */
> +    g_free(qtest_hmp(ahci->parent->qts, "qemu-io drive0 \"aio_flush\""));
> +
>      /* Round-trip through the device to confirm qemu is still alive. */
>      ahci_px_rreg(ahci, port, AHCI_PX_TFD);

Reviewed-by: Fabiano Rosas <farosas@suse.de>