[libvirt] [PATCH 2/2] Fix updating of haveTheBuck in RPC client to be race-free

Daniel P. Berrange berrange at redhat.com
Wed Dec 7 14:46:39 UTC 2011

From: "Daniel P. Berrange" <berrange at redhat.com>

When one thread passes the buck to another thread, it uses
virCondSignal to wake up the target thread. The variable
'haveTheBuck' is not updated in a race-free manner when
this occurs. The current thread sets it to false, and the
woken up thread sets it to true. There is a window where
a 3rd thread can come in and grab the buck.

Even if this didn't lead to crashes & deadlocks, this would
still result in unfairness in the buckpassing algorithm.

A better solution is to *never* set haveTheBuck to false
when we're passing the buck. Only set it to false when there
is no further thread waiting for the buck.

* src/rpc/virnetclient.c: Only set haveTheBuck to false
  if no thread is waiting
 src/rpc/virnetclient.c |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/rpc/virnetclient.c b/src/rpc/virnetclient.c
index e35521d..469c6a5 100644
--- a/src/rpc/virnetclient.c
+++ b/src/rpc/virnetclient.c
@@ -1265,6 +1265,7 @@ static void virNetClientIOEventLoopPassTheBuck(virNetClientPtr client, virNetCli
         tmp = tmp->next;
+    client->haveTheBuck = false;
     VIR_DEBUG("No thread to pass the buck to");
     if (client->wantClose) {
@@ -1593,10 +1594,11 @@ static int virNetClientIO(virNetClientPtr client,
         /* Grr, someone passed the buck onto us ... */
+    } else {
+        client->haveTheBuck = true;
     VIR_DEBUG("We have the buck %p %p", client->waitDispatch, thiscall);
-    client->haveTheBuck = true;
      * The buck stops here!
@@ -1624,8 +1626,6 @@ static int virNetClientIO(virNetClientPtr client,
         rv = -1;
-    client->haveTheBuck = false;
     VIR_DEBUG("All done with our call %p %p %d", client->waitDispatch, thiscall, rv);
     return rv;

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