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[Xen-devel] blktap race against xenstore startup

To: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] blktap race against xenstore startup
From: "Stephen C. Tweedie" <sct@xxxxxxxxxx>
Date: Thu, 28 Sep 2006 23:17:54 +0100
Cc: Julian Chesterfield <jac90@xxxxxxxxx>, Steven Rostedt <rostedt@xxxxxxxxxxx>, Andrew Warfield <andrew.warfield@xxxxxxxxxxxx>
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Hi all,

With the various blktap fixes I've recently posted, blktap runs
reliably... the *second* time we start xend.  First time, blktapctrl
just dies on init.

It turns out that get_dom_domid() is SEGVing.  It calls

        e = xs_directory(h, xth, "/local/domain", &num);

and then iterates over the results to find the domain with the right
name (in this case, "Domain-0", which should be easy to find!)  Trouble
is, it's racing with xenstore startup, and when it calls this the first
time, it gets back an ENOENT (easily seen on an strace.)  That returns
e=NULL, and everything falls apart.

I have "fixed" it locally with the following terrible hack:

+       for (i = 0; i < 10; i++) {
+               e = xs_directory(h, xth, "/local/domain", &num);
+               if (e)
+                       break;
+               sleep(1);
+       }
-       e = xs_directory(h, xth, "/local/domain", &num);
-       for (i = 0; (i < num) && (domid == NULL); i++) {
+       for (i = 0; e && (i < num) && (domid == NULL); i++) {

which just loops calling xs_directory() with a 1-second pause in between
until it returns something sensible.  

Ugh.  There has got to be a better way to synchronise with the initial
population of the dom0 information into xenstore, surely?  Has no other
component of the Xen stack ever seen this before?


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