Re: [RESEND PATCH v7 7/8] kernfs: Replace per-fs rwsem with hashed rwsems.

[Tejun Heo <tj@kernel.org>]

Hello,

On Mon, Mar 21, 2022 at 05:55:53PM +0000, Al Viro wrote:
> Why bother with rwsem, when we don't need anything blocking under it?
> DEFINE_RWLOCK instead of DEFINE_SPINLOCK and don't make it static.

Oh I mean, in case the common readers get way too hot, percpu_rwsem is a
relatively easy way to shift the burder from the readers to the writers. I
doubt we'll need that.

> kernfs_walk_ns() - this is fucking insane; on the surface, it needs to
> be exclusive due to the use of the same static buffer.  It uses that
> buffer to generate a pathname, *THEN* walks over it with strsep().
> That's an... interesting approach, for the lack of other printable
> terms - we walk the chain of ancestors, concatenating their names
> into a buffer and separating those names with slashes, then we walk
> that buffer, searching for slashes...  WTF?

It takes the @parent to walk string @path from. Where does it generate the
pathname?

> kernfs_rename_ns() - exclusive; that's where the tree topology gets
> changed.

This is the only true writer and it shouldn't be difficult to convert the
others to read lock w/ e.g. dynamic allocations or percpu buffers.

> So we can just turn that spinlock into rwlock, replace the existing
> uses with read_lock()/read_unlock() in kernfs_{name,path_from_node,get_parent}
> and with write_lock()/write_unlock() in the rest of fs/kernfs/dir.c,
> make it non-static, put extern into kernfs-internal.h and there you
> go...
> 
> Wait a sec; what happens if e.g. kernfs_path_from_node() races with
> __kernfs_remove()?  We do _not_ clear ->parent, but we do drop references
> that used to pin what it used to point to, unless I'm misreading that
> code...  Or is it somehow prevented by drain-related logics?  Seeing
> that it seems to be possible to have kernfs_path_from_node() called from
> an interrupt context, that could be delicate...

kernfs_remove() is akin to freeing of the node and all its descendants. The
caller shouldn't be racing that against any other operations in the subtree.

Thanks.

-- 
tejun