On Wed, Mar 04, 2015 at 11:18:21AM +0100, Alberto Garcia wrote: > On Tue, Mar 03, 2015 at 10:38:45AM -0600, Stefan Hajnoczi wrote: > > > > + qemu_mutex_init(&tg->lock); > > > + throttle_init(&tg->ts); > > > + QLIST_INIT(&tg->head); > > > + tg->refcount = 1; > > > + > > > + /* insert new entry in the list */ > > > + QTAILQ_INSERT_TAIL(&throttle_groups, tg, list); > > > > It is safest to hold tg->lock before adding the group to the > > list. This way there is a memory barrier and other threads will not > > access the group until we've finished adding it to the list. > > I think that the throttle_group_incref/unref calls are only made from > the QEMU main loop, and that's the only code that deals with the > throttle_groups list, so I don't see any chance for a race condition > there. > > Also, there's no way a different thread can have access to a group > before it's in the list, because the only way to get a group is to > retrieve it from the list. > > If it was possible for two threads to try to incref() the same group > we would need to make the whole function thread-safe, otherwise we > would have a situation where two threads can create two groups with > the same name because both think that it doesn't exist yet. > > I will anyway double-check if that's the case. Maybe it's a good idea > to protect both calls with a mutex anyway so we don't have to rely on > any assumptions?
The assumption is fine if it's documented.
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