[Xen-devel] [PATCH] libxenstore: fix threading bug which cause xend startup hang

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

I wrote:
> If a multithreaded caller creates a thread which calls xs_read_watch,
> before it has set any watches with xs_watch, the thread in
> xs_read_watch will enter read_message and sit reading the xenstored fd
> without the appropriate locks held.  Other threads can then
> concurrently read the xenstored fd, which naturally does not work very
> well.

While I was investigating this, I noticed that xs_fileno, which is
used by numerous applications to be able to select() waiting for
xenstore watches, always returns -1 and has done so since
7268:2144de6eabcc "Make libxenstore thread-safe" in October 2008.

This is because nothing ever creates the pipe implied by watch_pipe.
xs_fileno should do so (and care should be taken that locking is
appropriate, which needs some thought).

I won't send a patch for this right away as I want to see whether my
deadlock bugfix works, first.

Ian.

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