On Wed, 2021-01-06 at 10:38 +0800, Fox Chen wrote:

Hi Ian,

I am rethinking this problem. Can we simply use a global lock?

 In your original patch 5, you have a global mutex attr_mutex to
protect attr, if we change it to a rwsem, is it enough to protect
both
inode and attr while having the concurrent read ability?

like this patch I submitted. ( clearly, I missed __kernfs_iattrs
part,
but just about that idea )
https://lore.kernel.org/lkml/20201207084333.179132-1-foxhlchen@gmail.com/ (local)

I don't think so.

kernfs_refresh_inode() writes to the inode so taking a read lock
will allow multiple processes to concurrently update it which is
what we need to avoid.

It's possibly even more interesting.

For example, kernfs_iop_rmdir() and kernfs_iop_mkdir() might alter
the inode link count (I don't know if that would be the sort of thing
they would do but kernfs can't possibly know either). Both of these
functions rely on the VFS locking for exclusion but the inode link
count is updated in kernfs_refresh_inode() too.

That's the case now, without any patches.

I'm not entirely sure what's going on in kernfs_refresh_inode().

It could be as simple as being called with a NULL inode because
the dentry concerned is negative at that point. I haven't had
time to look closely at it TBH but I have been thinking about it.

Ian

thanks,
fox