On 07/31/2018 07:04 PM, Andrey Ryabinin wrote: >> Somewhat offtopic, but I can't understand how SLAB_TYPESAFE_BY_RCU >> slabs can be useful without ctors or at least memset(0). Objects in >> such slabs need to be type-stable, but I can't understand how it's >> possible to establish type stability without a ctor... Are these bugs? > > Yeah, I puzzled by this too. However, I think it's hard but possible to make it work, at least in theory. > There must be an initializer, which consists of two parts: > a) initilize objects fields > b) expose object to the world (add it to list or something like that) > > (a) part must somehow to be ok to race with another cpu which might already use the object. > (b) part must must use e.g. barriers to make sure that racy users will see previously inilized fields. > Racy users must have parring barrier of course. > > But it sound fishy, and very easy to fuck up. I won't be surprised if every single one SLAB_TYPESAFE_BY_RCU user > without ->ctor is bogus. It certainly would be better to convert those to use ->ctor. > > Such caches seems used by networking subsystem in proto_register(): > > prot->slab = kmem_cache_create_usercopy(prot->name, > prot->obj_size, 0, > SLAB_HWCACHE_ALIGN | SLAB_ACCOUNT | > prot->slab_flags, > prot->useroffset, prot->usersize, > NULL); > > And certain protocols specify SLAB_TYPESAFE_BY_RCU in ->slab_flags, such as: > llc_proto, smc_proto, smc_proto6, tcp_prot, tcpv6_prot, dccp_v6_prot, dccp_v4_prot. > > > Also nf_conntrack_cachep, kernfs_node_cache, jbd2_journal_head_cache and i915_request cache. > [+CC maintainer of the relevant code.] 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?
