Re: Vnode API change: add global vnode cache

["J. Hannken-Illjes" <hannken%eis.cs.tu-bs.de@localhost>]

On 07 Apr 2014, at 18:38, Taylor R Campbell 
<campbell+netbsd-tech-kern%mumble.net@localhost> wrote:

>   Date: Mon, 7 Apr 2014 18:32:02 +0200
>   From: "J. Hannken-Illjes" <hannken%eis.cs.tu-bs.de@localhost>
> 
>   On 07 Apr 2014, at 18:02, Taylor R Campbell 
> <campbell+netbsd-tech-kern%mumble.net@localhost> wrote:
> 
>> In that case, could you set the VI_CHANGING bit in vp, rather than
>> node->vn_vnode = NULL in the vcache node, to indicate that the vnode
>> is being initialized in vcache_intern?
> 
>   This will not work for layered file systems.  They will share the
>   v_interlock from another vnode (in VFS_LOAD_NODE) so we better don't use
>   the interlock before loading the fs node.
> 
> You can pass the shared interlock to vcache_intern, like I illustrated
> in the code I attached.

Sorry, I missed the new argument to vcache_intern.  But there is still
a window between getnewvnode() and setting VI_CHANGING where another
thread may succeed with vget() so for this to work either getnewvnode()
returns with interlock held or it has to set VI_CHANGING and all callers
of getnewvnode have to be changed.

>> Perhaps we ought at least to change VFS_VGET so it returns the vnode
>> unlocked, and once we do that, use it consistently to look up vnodes
>> by inode number rather than calling vcache_intern in multiple
>> different places.
> 
>   Maybe in the far future.  Already looked at it and it is a very big job
>   if we want to do it in one step.
> 
> We could do this for just ufs:
> 
> 1. Add ufs_vget_unlocked to make the sole call to vcache_intern for
> ufs.
> 
> 2. Make ufs_vget call ufs_vget_unlocked and then lock the result.
> 
> 3. Replace calls to VFS_VGET by ufs_vget_unlocked, rather than by
> vcache_intern.
> 
> Then, when we fix VFS_VGET, we replace ufs_vget by ufs_vget_unlocked.

Ok -- tomorrow ...

--
J. Hannken-Illjes - hannken%eis.cs.tu-bs.de@localhost - TU Braunschweig 
(Germany)