Re: [Qemu-devel] another locking issue in current dataplane code?

[Paolo Bonzini <pbonzini@redhat.com>]

Il 08/07/2014 17:59, Stefan Hajnoczi ha scritto:
> I sent Christian an initial patch to fix this but now both threads are
> stuck in rfifolock_lock() inside cond wait.  That's very strange and
> should never happen.

I had this patch pending for 2.2:

commit 6c81e31615c3cda5ea981a998ba8b1b8ed17de6f
Author: Paolo Bonzini <pbonzini@redhat.com>
Date:   Mon Jul 7 10:39:49 2014 +0200

    iothread: do not rely on aio_poll(ctx, true) result to end a loop
    
    Currently, whenever aio_poll(ctx, true) has completed all pending
    work it returns true *and* the next call to aio_poll(ctx, true)
    will not block.
    
    This invariant has its roots in qemu_aio_flush()'s implementation
    as "while (qemu_aio_wait()) {}".  However, qemu_aio_flush() does
    not exist anymore and bdrv_drain_all() is implemented differently;
    and this invariant is complicated to maintain and subtly different
    from the return value of GMainLoop's g_main_context_iteration.
    
    All calls to aio_poll(ctx, true) except one are guarded by a
    while() loop checking for a request to be incomplete, or a
    BlockDriverState to be idle.  Modify that one exception in
    iothread.c.
    
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/iothread.c b/iothread.c
index 1fbf9f1..d9403cf 100644
--- a/iothread.c
+++ b/iothread.c
@@ -30,6 +30,7 @@ typedef ObjectClass IOThreadClass;
 static void *iothread_run(void *opaque)
 {
     IOThread *iothread = opaque;
+    bool blocking;
 
     qemu_mutex_lock(&iothread->init_done_lock);
     iothread->thread_id = qemu_get_thread_id();
@@ -38,8 +39,10 @@ static void *iothread_run(void *opaque)
 
     while (!iothread->stopping) {
         aio_context_acquire(iothread->ctx);
-        while (!iothread->stopping && aio_poll(iothread->ctx, true)) {
+        blocking = true;
+        while (!iothread->stopping && aio_poll(iothread->ctx, blocking)) {
             /* Progress was made, keep going */
+            blocking = false;
         }
         aio_context_release(iothread->ctx);
     }

Christian, can you test it?

Paolo