[libvirt] [PATCH 2/2] Fix updating of haveTheBuck in RPC client to be race-free
From: "Daniel P. Berrange" <berrange(a)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,
virGetLastError())
rv = -1;
- client->haveTheBuck = false;
-
cleanup:
VIR_DEBUG("All done with our call %p %p %d", client->waitDispatch,
thiscall, rv);
return rv;
--
1.7.7.3