Re: Urgent help needed on an NFS question, please help!!!

["Xin Zhao" <uszhaoxin@gmail.com>]

Many thanks for your kind help!

Your answer is what I expected. But what frustrated me is that I
cannot find the code that verifies the generation number in NFS V3
codes. Do you know where it check the generation number?

Thanks,
-x

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
>