On Tuesday 09 September 2008 06:09:34 am Robert Watson wrote: > On Mon, 8 Sep 2008, John Baldwin wrote: > > > SVN rev 182879 on 2008-09-08 21:40:15Z by jhb > > > > - Reduce scope of #ifdef's in uma_zcreate() call in init_turnstile0(). > > - Set UMA_ZONE_NOFREE so that the per-turnstile spin locks are type stable > > to avoid a race where one thread might dereference a lock in a free'd > > turnstile that was previously used by another thread. > > Is this a feature or a workaround for a bug? Normally in the above scenario > we would consider use-after-free a bug or symptom of a larger architectural > problem rather than a feature. At least, that's what I consider similar use > of UMA_ZONE_NOFREE where it persists in the network stack :-).
Well, it's a workaround for the fact that the way thread_lock works is it tries to acquire what it thinks is the current lock for a given thread. Once it has that lock, then it checks to see if the thread has switched to a different lock. If so, it drops the lock it has and tries to get the "new" lock. Anytime you lose this race, you can end up holding a lock that isn't necessarily associated with the thread anymore. For that reason, locks used as thread locks should generally be type-stable. Most of the locks used as thread locks are in static data structures (runqueues, sleepq hash table buckets, the global "blocked lock", etc.) so they are already type-stable. The turnstile locks are the one case where locks used as thread locks are dynamically allocated IIRC. -- John Baldwin _______________________________________________ [email protected] mailing list http://lists.freebsd.org/mailman/listinfo/cvs-all To unsubscribe, send any mail to "[EMAIL PROTECTED]"
