[libvirt] [PATCH 3/3] Avoid rare race when undefining domain

[Michal Privoznik]

On 30.10.2014 16:04, Martin Kletzander wrote:
> When one domain is being undefined and at the same time started, for
> example, there is a possibility of a rare problem occuring.
>
>   - Thread 1 does virDomainUndefine(), has the lock, checks that the
>     domain is active and because it's not, calls
>     virDomainObjListRemove().
>
>   - Thread 2 does virDomainCreate() and tries to lock the domain.
>
>   - Thread 1 needs to lock domain list in order to remove the domain from
>     it, but must unlock domain first (proper order is to lock domain list
>     first and the domain itself second).
>
>   - Thread 2 grabs the lock, starts the domain and releases the lock.
>
>   - Thread 1 grabs the lock and removes the domain from list.
>
> With this patch:
>
>   - The undefining domain gets marked as "to undefine" before it is
>     unlocked.
>
>   - If domain is found in any of the search APIs, it's returned only if
>     it is not marked as "to undefine".  The check is done while the
>     domain is locked.
>
> Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1150505
>
> Signed-off-by: Martin Kletzander <mkletzan at redhat.com>
> ---
>   src/conf/domain_conf.c | 23 ++++++++++++++++++++---
>   src/conf/domain_conf.h |  1 +
>   2 files changed, 21 insertions(+), 3 deletions(-)
>
> diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
> index 43574e1..b92a58a 100644
> --- a/src/conf/domain_conf.c
> +++ b/src/conf/domain_conf.c
> @@ -1054,8 +1054,13 @@ virDomainObjPtr virDomainObjListFindByID(virDomainObjListPtr doms,
>       virDomainObjPtr obj;
>       virObjectLock(doms);
>       obj = virHashSearch(doms->objs, virDomainObjListSearchID, &id);
> -    if (obj)
> +    if (obj) {
>           virObjectLock(obj);
> +        if (obj->undefining) {
> +            virObjectUnlock(obj);
> +            obj = NULL;
> +        }
> +    }
>       virObjectUnlock(doms);
>       return obj;
>   }

I find this too hackish, Wouldn't it be better to hold the domain list 
locked during the whole undefine procedure? On one hand the throughput 
would get hurt but on the other, the code would be cleaner. Or maybe we 
can just make the Undefine() API to grab the QEMU_JOB_MODIFY job which 
would make the APIs serialize.

Michal