Re: [PATCH 1/1] fuse: Make atomic_open use negative d_entry

[Yuan Yao <yuanyaogoog@xxxxxxxxxxxx>]

Hi Bernd,

Thank you for taking a look at this patch! I appreciate it a lot for
adding this patch to the next version. However, I just found that
there’s a bug with my patch. The *BUG_ON(!d_unhashed(dentry));* in
d_splice_alias() function will be triggered when doing the parallel
lookup on a non-exist file.

> struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
> {
>    if (IS_ERR(inode))
>        return ERR_CAST(inode);
>
>    BUG_ON(!d_unhashed(dentry));

This bug can be easily reproduced by adding 3 seconds sleep in fuse
server’s atomic_open handler, and execute the following commands in
fuse filesystem:
cat non-exist-file &
cat non-exist-file &

I think this bug can be addressed by following two approaches:

1. adding check for d_in_lookup(entry)
-----------------------------------------------------------------------
       if (outentry.entry_valid) {
+            if (d_in_lookup(entry)) {
                inode = NULL;
                d_splice_alias(inode, entry);
               fuse_change_entry_timeout(entry, &outentry);
+          }
            goto free_and_no_open;
        }

2. adding d_drop(entry)
-----------------------------------------------------------------------
        if (outentry.entry_valid) {
             inode = NULL;
+           d_drop(entry)
             d_splice_alias(inode, entry);
             fuse_change_entry_timeout(entry, &outentry);
            goto free_and_no_open;
        }

But I'm not so sure which one is preferable, or maybe the handling
should be elsewhere?

Best,
Yuan