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rpc: fix race in waking up client event loop

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The first thread to issue a client RPC request will own the event
loop execution, sitting in the virNetClientIOEventLoop function.

It releases the client lock while running:

   virNetClientUnlock()
   g_main_loop_run()
   virNetClientLock()

If a second thread arrives with an RPC request, it will queue it
for the first thread to process. To inform the first thread that
there's a new request it calls g_main_loop_quit() to break it out
of the main loop.

This works if the first thread is in g_main_loop_run() at that
time. There is a small window of opportunity, however, where
the first thread has released the client lock, but not yet got
into g_main_loop_run(). If that happens, the wakeup from the
second thread is lost.

This patch deals with that by changing the way the wakeup is
performed. Instead of directly calling g_main_loop_quit(), the
second thread creates an idle source to run the quit function
from within the first thread. This guarantees that the first
thread will see the wakeup.

Tested by: Fima Shevrin <efim.shevrin@virtuozzo.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
This commit is contained in:
Daniel P. Berrangé 2023-12-18 12:17:18 +00:00
parent 024d6dc263
commit 7cb03e6a28
1 changed files with 12 additions and 1 deletions
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13
src/rpc/virnetclient.c
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@ -1848,6 +1848,15 @@ static void virNetClientIOUpdateCallback(virNetClient *client,
}
static gboolean virNetClientIOWakeup(gpointer opaque)
{
GMainLoop *loop = opaque;
g_main_loop_quit(loop);
return G_SOURCE_REMOVE;
}
/*
* This function sends a message to remote server and awaits a reply
*
@ -1925,7 +1934,9 @@ static int virNetClientIO(virNetClient *client,
/* Check to see if another thread is dispatching */
if (client->haveTheBuck) {
/* Force other thread to wakeup from poll */
g_main_loop_quit(client->eventLoop);
GSource *wakeup = g_idle_source_new();
g_source_set_callback(wakeup, virNetClientIOWakeup, client->eventLoop, NULL);
g_source_attach(wakeup, client->eventCtx);
/* If we are non-blocking, detach the thread and keep the call in the
* queue. */