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SubjectRe: [RFC PATCH 1/4] Union mount documentation.
On Tue, 19 Jun 2007 22:59:51 -0700, Arjan van de Ven wrote:

> user does on FS A:
> mkdir /mnt/A/somedir
> touch /mnt/A/somedir/somefile
> and then 2 things happen in parallel
> 1) touch /mnt/B/somefile
> 2) mv /mnt/union/somedir /mnt/union/somefile
> since the underlying FS for 2) is FS A... how will this work out locking
> wise? Will the VS lock the union directory only? Or will this operate
> only on the underlying FS? How is dcache consistency guaranteed for
> scenarios like this?

Ok, with Christophs help I guess I know now what the question is :)

touch /mnt/B/somefile is doing a lookup in "B" for "somefile". Therefore it
locks B->i_mutex for that. When it gets a negative dentry it creates the

mv /mnt/union/somedir /mnt/union/somefile is doing a lookup in "union" for
"somefile". Therefore it first locks the i_mutex of the topmost directory
in the union of "/mnt/union" (which happens to be "B"). When it gets a
negative dentry it than follows the union down to the next layer (with the
topmost directory still locked). Lookup is repeated until a filled dentry
is found or the topmost dentry negative dentry is used as a target for the
move. Thats it.

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