On Thu, 3 Sep 2009, Paul E. McKenney wrote: > 2. CPU 0 discovers that the slab cache can now be destroyed. > > It determines that there are no users, and has guaranteed > that there will be no future users. So it knows that it > can safely do kmem_cache_destroy(). > > 3. In absence of rcu_barrier(), kmem_cache_destroy() would > immediately tear down the slab data structures. Of course. This has been discussed before. You need to ensure that no objects are in use before destroying a slab. In case of DESTROY_BY_RCU you must ensure that there are no potential readers. So use a suitable rcu barrier or something else like a synchronize_rcu... > > But going through the RCU period is pointless since no user of the cache > > remains. > > Which is irrelevant. The outstanding RCU callback was posted by the > slab cache itself, -not- by the user of the slab cache. There will be no rcu callbacks generated at kmem_cache_destroy with the patch I posted. > > The dismantling does not need RCU since there are no operations on the > > objects in progress. So simply switch DESTROY_BY_RCU off for close. > > Unless I am missing something, this patch re-introduces the bug that > the rcu_barrier() was added to prevent. So, in absence of a better > explanation of what I am missing: The "fix" was ill advised. Slab users must ensure that no objects are in use before destroying a slab. Only the slab users know how the objects are being used. The slab allocator itself cannot know how to ensure that there are no pending references. Putting a rcu_barrier in there creates an inconsistency in the operation of kmem_cache_destroy() and an expectation of functionality that the function cannot provide. -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
