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    SubjectRe: Urgent help needed on an NFS question, please help!!!
    I think nfs_compare_fh() might do the file handle verification task.
    However, it is still possible that AFTER C1 gets a valid file handle,
    BUT BEFORE C1 sends out the getattr() request, C2 deletes file X and
    creates a different file X1 which has the same inode number. Looks
    like the server side must verify the generation number carried in the
    file handle. Unfortunately, I didn't find this code at the server
    side. Any further insight on this?

    Thanks,
    Xin

    On 8/10/06, Neil Brown <neilb@suse.de> wrote:
    > On Thursday August 10, uszhaoxin@gmail.com wrote:
    > > I just ran into a problem about NFS. It might be a fundmental problem
    > > of my current work. So please help!
    > >
    > > I am wondering how NFS guarantees a client didn't get wrong file
    > > attributes. Consider the following scenario:
    > >
    > > Suppose we have an NFS server S and two clients C1 and C2.
    > >
    > > Now C1 needs to access the file attributes of file X, it first does
    > > lookup() to get the file handle of file X.
    > >
    > > After C1 gets X's file handle and before C1 issues the getattr()
    > > request, C2 cuts in. Now C2 deletes file X and creates a new file X1,
    > > which has different name but the same inode number and device ID as
    > > the nonexistent file X.
    > >
    > > When C1 issues getattr() with the old file handle, it may get file
    > > attribute on wrong file X1. Is this true?
    > >
    > > If not, how NFS avoid this problem? Please direct me to the code that
    > > verifies this.
    >
    > Generation numbers.
    >
    > When the filesystem creates a new file it assigns a random number
    > as the 'generation' number and stores that in the inode.
    > This gets included in the filehandle, and checked when the filehandle
    > lookup is done.
    >
    > Look for references to 'i_generation' in fs/ext3/*
    >
    > Other files systems may approach this slightly differently, but the
    > filesystem is responsible for providing a unique-over-time filehandle,
    > and 'generation number' is the 'standard' way of doing this.
    >
    > NeilBrown
    >
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