On Thu, 12 Jun 2025, Al Viro wrote: > On Thu, Jun 12, 2025 at 08:57:03AM +1000, NeilBrown wrote: > > > However there is no guarantee that this lock is held by d_same_name() > > (the caller of ->d_compare). In particularly d_alloc_parallel() calls > > d_same_name() after rcu_read_unlock(). > > d_alloc_parallel() calls d_same_name() with dentry being pinned; > if it's positive, nothing's going to happen to its inode, > rcu_read_lock() or not. It can go from negative to positive, > but that's it. > > Why is it needed? We do care about possibly NULL inode (basically, > when RCU dcache lookup runs into a dentry getting evicted right > under it), but that's not relevant here. > Maybe it isn't needed. Maybe I could fix the warning by removing the rcu_dereference() (and the RCU_INIT_POINTER() in inode.c). But then I might have to pretend that I understand the code - and it makes no sense. If a second d_alloc_parallel() is called while there is already a d_in_lookup() dentry, then ->d_compare will return 1 so a second d_in_lookup() will be created and ->lookup will be called twice (possibly concurrently) and both will be added to the dcache. Probably not harmful but not really wanted. And I'm having trouble seeing how sysctl_is_seen() is useful. If it reports that the sysctl is not visible to this process, it'll just create a new dentry/inode which is that same as any other that would be created... NeilBrown
