Re: [RFC PATCH 06/19] rust: fs: introduce `FileSystem::init_root`

[Dave Chinner <david@fromorbit.com>]

On Sat, Oct 21, 2023 at 12:33:57PM -0700, Boqun Feng wrote:
> On Sat, Oct 21, 2023 at 06:01:02PM +0100, Matthew Wilcox wrote:
> > I'm only an expert on the page cache, not the rest of the VFS.  So
> > what are the rules around modifying i_state for the VFS?
> 
> Agreed, same question here.

inode->i_state should only be modified under inode->i_lock.

And in most situations, you have to hold the inode->i_lock to read
state flags as well so that reads are serialised against
modifications which are typically non-atomic RMW operations.

There is, I think, one main exception to read side locking and this
is find_inode_rcu() which does an unlocked check for I_WILL_FREE |
I_FREEING. In this case, the inode->i_state updates in iput_final()
use WRITE_ONCE under the inode->i_lock to provide the necessary
semantics for the unlocked READ_ONCE() done under rcu_read_lock().

IOWs, if you follow the general rule that any inode->i_state access
(read or write) needs to hold inode->i_lock, you probably won't
screw up. 

-Dave.
-- 
Dave Chinner
david@fromorbit.com