Re: [Qemu-devel] [PATCH v3 1/3] blkdebug: fix one shot rule processing

[John Snow <jsnow@redhat.com>]

On 11/12/18 2:06 AM, Marc Olson via Qemu-devel wrote:
> If 'once' is specified, the rule should execute just once, regardless if
> it is supposed to return an error or not. Take the example where you
> want the first IO to an LBA to succeed, but subsequent IOs to fail. You
> could either use state transitions, or create two rules, one with
> error = 0 and once set to true, and one with a non-zero error.
> 
> Signed-off-by: Marc Olson <marcolso@amazon.com>
> ---
>  block/blkdebug.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blkdebug.c b/block/blkdebug.c
> index 0759452..327049b 100644
> --- a/block/blkdebug.c
> +++ b/block/blkdebug.c
> @@ -488,7 +488,7 @@ static int rule_check(BlockDriverState *bs, uint64_t offset, uint64_t bytes)
>          }
>      }
>  
> -    if (!rule || !rule->options.inject.error) {
> +    if (!rule) {
>          return 0;
>      }
>  

This gets rid of the early return so that later we check to see if
'once' was set and remove the rule, regardless of if it did anything or not,

> @@ -500,7 +500,7 @@ static int rule_check(BlockDriverState *bs, uint64_t offset, uint64_t bytes)
>          remove_rule(rule);
>      }
>  
> -    if (!immediately) {
> +    if (error && !immediately) {

And then we modify this to only trigger if we have an error to inject.

>          aio_co_schedule(qemu_get_current_aio_context(), qemu_coroutine_self());
>          qemu_coroutine_yield();
>      }
> 

[down here, we return -error, but that should still be zero.]


This changes the mechanism of 'once' slightly, but only when errno was
set to zero. I'm not sure we make use of that anywhere, so I think this
should be a safe change. Certainly we don't stipulate that we only
respect once if you bothered to set errno to a non-zero value.

I thiink this is probably fine.

Reviewed-by: John Snow <jsnow@redhat.com>