Re: [libvirt-users] error : virHashForEach:597 : Hash operation not allowed during iteration

Vincent Bernat
Wed, 04 Apr 2018 18:38:12 -0000

❦  4 avril 2018 17:00 +0200, Vincent Bernat <bernat@luffy.cx> :

>>> You can do that locally, but as a patch it's very unlikely to
be
>>> accepted upstream because we've introduced RW locks to be able
to access
>>> domain list from multiple threads.
>>
>> Looking a bit more, the whole "iterating" bit is currently
>> unsafe. First, it is racy since it's "check then take".
Two threads may
>> check the value is false and start iterating both. Moreover, at some
>> places, it is not set, for example in virHashAddOrUpdateEntry(). So, it
>> would be possible to call this function and during the update to
iterate
>> over the hash. This seems to be prevented only by callers using RW
>> lock. So, it seems we can just remove this "iterating" bool
and just
>> check all users of these functions are using the appropriate lock.
>
> My bad, I didn't see the use of table->current...

It seems it can also go away if caller correctly use a RW lock. I am
currently testing the attached patch on a few servers (patch for master,
but applied on our 3.6 version). After checking many locations, I came
to conclusion that locks are used correctly.




--
Use library functions.
            - The Elements of Programming Style (Kernighan & Plauger)