Re: [PATCH 1/4] vfio: ccw: fix cleanup if cp_prefetch fails

Halil Pasic
2018-03-21T12:49:54Z

On 03/21/2018 03:08 AM, Dong Jia Shi wrote:
> From: Halil Pasic <pasic@linux.vnet.ibm.com>
>
> If the translation of a channel program fails, we may end up attempting
> to clean up (free, unpin) stuff that never got translated (and allocated,
> pinned) in the first place.
>
> By adjusting the lengths of the chains accordingly (so the element that
> failed, and all subsequent elements are excluded) cleanup activities
> based on false assumptions can be avoided.
>
> Let's make sure cp_free works properly after cp_prefetch returns with an
> error by setting ch_len to 0 for the ccw chains those are not prefetched.

This sentence used to be:

Let's make sure cp_free works properly after cp_prefetch returns with an
error.

@Dong Jia
I find the 'by setting ch_len to 0 for the ccw chains those are not prefetched'
you added for clarification (I guess) somewhat problematic.
The chain in which the translation failure occurred
+ chain->ch_len = idx;
is shortened so that only the translated elements (ccws) are going to
get cleaned up (on a per element basis) by cp_free. This may or may
not be the first ccw. Subsequent chains are shortened to 0 as there
no translation took place.

So as a result of this change only properly translated ccws are going
to get (re)visited by cp_free as only those may have resources bound
to them which need to be released.

I'm not against improving the commit message. But this ain't
an improvement to me.

>
> Acked-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
> Reviewed-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
> Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
> Signed-off-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
> ---
> drivers/s390/cio/vfio_ccw_cp.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/s390/cio/vfio_ccw_cp.c b/drivers/s390/cio/vfio_ccw_cp.c
> index d9a2fffd034b..2be114db02f9 100644
> --- a/drivers/s390/cio/vfio_ccw_cp.c
> +++ b/drivers/s390/cio/vfio_ccw_cp.c
> @@ -749,11 +749,18 @@ int cp_prefetch(struct channel_program *cp)
> for (idx = 0; idx < len; idx++) {
> ret = ccwchain_fetch_one(chain, idx, cp);
> if (ret)
> - return ret;
> + goto out_err;
> }
> }
>
> return 0;
> +out_err:
> + /* Only cleanup the chain elements that where actually translated. */
> + chain->ch_len = idx;
> + list_for_each_entry_continue(chain, &cp->ccwchain_list, next) {
> + chain->ch_len = 0;
> + }
> + return ret;
> }
>
> /**
>