On Tue 15 Aug 2017 03:05:02 PM CEST, Stefan Hajnoczi wrote:
> The 093 throttling test submits twice as many requests as the throttle
> limit in order to ensure that we reach the limit. The remaining
> requests are left in-flight at the end of each test iteration.
>
> Commit 452589b6b47e8dc6353df257fc803dfc1383bed8 ("vl.c/exit: pause cpus
> before closing block devices") exposed a hang in 093. This happens
> because requests are still in flight when QEMU terminates but
> QEMU_CLOCK_VIRTUAL time is frozen. bdrv_drain_all() hangs forever since
> throttled requests cannot complete.
>
> Step the clock at the end of each test iteration so in-flight requests
> actually finish. This solves the hang and is cleaner than leaving tests
> in-flight.
>
> Note that this could also be "fixed" by disabling throttling when drives
> are closed in QEMU. That approach has two issues:
>
> 1. We must drain requests before disabling throttling, so the hang
> cannot be easily avoided!
>
> 2. Any time QEMU disables throttling internally there is a chance that
> malicious users can abuse the code path to bypass throttling limits.
>
> Therefore it makes more sense to fix the test case than to modify QEMU.
>
> Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Alberto Garcia <[email protected]>
Berto
