On Mon, Jul 13, 2020 at 10:25 PM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote: > > On Mon, Jul 13, 2020 at 01:57:31PM +0300, Amir Goldstein wrote: > > Changes to underlying layers while overlay in mounted result in > > undefined behavior. Therefore, we can change the behavior to > > invalidate the overlay dentry on dcache lookup if one of the > > underlying dentries was deleted since the dentry was composed. > > > > Negative underlying dentries are not expected in overlay upper and > > lower dentries. If they are found it is probably dcache lookup racing > > with an overlay unlink, before d_drop() was called on the overlay dentry. > > IS_DEADDIR directories may be caused by underlying rmdir, so invalidate > > overlay dentry on dcache lookup if we find those. > > Can you elaborate a bit more on this race. Doesn't inode_lock_nested(dir) > protect against that. I see that both vfs_rmdir() and vfs_unlink() > happen with parent directory inode mutex held exclusively. And IIUC, > that should mean no further lookup()/->revalidate() must be in progress > on that dentry? I might very well be wrong, hence asking for more > details. > lookup_fast() looks in dcache without dir inode lock. d_revalidate() is called to check if the found cached dentry is valid. For example, ovl_remove_upper() can make an upper dentry negative or upper dir inode S_DEAD (i.e. vfs_rmdir) just before calling d_drop() to prevent overlay dentry from being found in fast cache lookup. Unless I am missing something, that leaves a small window where lookup_fast() can return an overlay dentry with negative/S_DEAD upper dentry, which was not caused by illegitimate underlying fs changes, so we must gracefully invalidate the dcache lookup (return 0 from revalidate) in order to fallback to fs lookup. Thanks, Amir.
