d_add on negative dentry?

[Urban Widmark]

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Is it valid to call d_add on a negative dentry?
(or on a dentry that is already linked in d_hash, but all negative
 dentries are, right?)

I'm guessing it isn't because I think that is how I can get my machine to
hang in d_lookup, looping on a corrupt d_hash list.

The problem can be reproduced like this. /mnt/smb is a smbfs mount of
/mnt/samba/export from a samba server on localhost.

/mnt/smb% ls
aa
/mnt/smb% rm aa
/mnt/smb% touch /mnt/samba/export/aa
/mnt/smb% ls
ls: aa: No such file or directory

And shortly after it will lock up completely.

My printk's tell me that a negative dentry is still hashed since d_hash is
non-empty. d_add calls d_instantiate and d_rehash, the later adds it to a
d_hash list without first removing it. But it was already linked so now 2
extra dentries are also pointing to this dentry. And it is then no longer
a list ...

The attached patch fixes things for me. Comments?

Oh, and I *think* ncpfs has the same problem. But that's just from reading
the code.

/Urban

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