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@xxxxxxxxxxxxx
