On Tue, Jul 31, 2018 at 10:10 AM Florian Westphal <fw@xxxxxxxxx> wrote: > > Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> 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: > 1. compare keys > 2. attempt to obtain refcount (using _not_zero version) > 3. 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.
