On Tue, Jul 31, 2018 at 10:10 AM Florian Westphal fw@strlen.de wrote:
Andrey Ryabinin aryabinin@virtuozzo.com wrote:
Guys, it seems that we have a lot of code using SLAB_TYPESAFE_BY_RCU cache without constructor. I think it's nearly impossible to use that combination without having bugs. It's either you don't really need the SLAB_TYPESAFE_BY_RCU, or you need to have a constructor in kmem_cache.
Could you guys, please, verify your code if it's really need SLAB_TYPSAFE or constructor?
E.g. the netlink code look extremely suspicious:
/* * Do not use kmem_cache_zalloc(), as this cache uses * SLAB_TYPESAFE_BY_RCU. */ ct = kmem_cache_alloc(nf_conntrack_cachep, gfp); if (ct == NULL) goto out; spin_lock_init(&ct->lock);If nf_conntrack_cachep objects really used in rcu typesafe manner, than 'ct' returned by kmem_cache_alloc might still be in use by another cpu. So we just reinitialize spin_lock used by someone else?
That would be a bug, nf_conn objects are reference counted.
spinlock can only be used after object had its refcount incremented.
lookup operation on nf_conn object:
- compare keys
- attempt to obtain refcount (using _not_zero version)
- compare keys again after refcount was obtained
if any of that fails, nf_conn candidate is skipped.
Yes, the key here is the refcount, this is only what we need to clear after kmem_cache_alloc()
By definition, if an object is being freed/reallocated, the refcount should be already 0, and clearing it again is a NOP.