Re: [libvirt] [PATCH v5 1/3] nwfilter: Convert _virNWFilterObj to use virObjectRWLockable

[...] >>>> +static void >>>> +virNWFilterObjPromoteToWrite(virNWFilterObjPtr obj) >>>> +{ >>>> + virObjectRWUnlock(obj); >>>> + virObjectRWLockWrite(obj); >>>> +} >>> >>> How can this not deadlock? This will work only if @obj is locked exactly >>> once. But since we allow recursive locks any lock() that happens in the >>> 2nd level must deadlock with this. On the other hand, there's no such >>> case in the code currently. But that doesn't stop anybody from calling >>> PromoteWrite() without understanding its limitations. >>> >>> Maybe the fact that we need recursive lock for NWFilterObj means it's >>> not a good candidate for virObjectRWLocable? It is still a good >>> candidate for virObject though. >>> >>> Or if you want to spend extra time on this, maybe introducing >>> virObjectRecursiveLockable may be worth it (terrible name, I know). >>> >>> >> >> I dunno, worked for me. The helper is being called from a thread that >> has taken out a READ lock at most one time and needs to get the WRITE >> lock in order to change things. If something else has the READ lock that >> thread waits until the other thread releases the READ lock as far as I >> understand things. > > Yes, I can see that. It's just that since the original lock is recursive > I expected things to get recursive and thus I've been thinking how would > this work there, nested in 2nd, 3rd, .. level. But as noted earlier, the > places where lock promoting is used are kind of safe since all the > locking is done at the first level. > The reason I chose rwlocks and taking a read lock by default in the FindByUUID and FindByName (as opposed to other vir*FindBy* API's which return a write locked object) is because I know there's the potential for other code to be doing Reads and rwlocks allowed for that without the need to create any sort of recursive lockable object or any sort of API to perform "trylock"'s (all things I tried until the epiphany to use rwlocks was reached when you added them for other drivers and their list protection). In the long/short run I figured that having a common way to get the lock and then deciding to change it from Read to Write as necessary would work just fine for the purpose that was needed.

>> >> Back when I first did this I had quite a lot of debug code and I have a >> vague recollection of seeing output from this particular path and seeing >> a couple of unlocks before the WRITE was taken while running the avocado >> tests. >> >> I have zero interest in spending more time on this. That ship sailed. If >> the decision is to drop the patches, then fine. I tried. I disagree >> that NWFilterObj is not a candidate for RWLockable - in fact it's quite >> the opposite *because* of the recursive locks. > > Well, there's a difference between recursive locks and rwlocks. And it's > exactly in what happens in recursion. With recursive locks we don't need > any lock promoting and thus it's safe to do read/write after lock(). > With lock promoting (esp. done in unlock()+lockWrite() manner) we get > TOCTOU bugs. > If we had a real lock manager available this wouldn't be a problem ;-) [sorry just my OpenVMS roots and bias showing]. Still if you read up a bit on "dlm" you'll get an idea of what I mean. There's so many locks in the nwfilter code it's a wonder it all works.

It's truly unfortunate that the wrlock mechanism is undefined for promoting a read lock to write leaving the consumer with few options.

Since we can limit the "updating" of the fields in @obj and getting a write lock to very specific code paths, hopefully we can limit damage and/or deadlock type and/or TOCTOU issues