On Mon, Jan 11, 2021 at 12:20 PM Ian Kent <raven@xxxxxxxxxx> wrote: > > On Mon, 2021-01-11 at 11:19 +0800, Ian Kent wrote: > > 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@xxxxxxxxx/ > > > > 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. Oh, got it. I missed the inode part. my bad. :( > > 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. > > So it's not so easy to get the inode from just the kernfs object > so these probably aren't a problem ... IIUC only when dop->revalidate, iop->lookup being called, the result of rmdir/mkdir will be sync with vfs. kernfs_node is detached from vfs inode/dentry to save ram. > > > > 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. um, It shouldn't be called with a NULL inode, right? inode->i_mode = kn->mode; otherwise will crash. > Certainly this can be called without a struct iattr having been > allocated ... and given it probably needs to remain a pointer > rather than embedded in the node the inode link count update > can't easily be protected from concurrent updates. > > If it was ok to do the allocation at inode creation the problem > becomes much simpler to resolve but I thought there were concerns > about ram consumption (although I don't think that was exactly what > was said?). > you meant iattr to be allocated at inode creation time?? yes, I think so. it's due to ram consumption. thanks, fox
