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

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 08/07/14 19:08, Paolo Bonzini wrote:
> 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>

The hangs are gone. Looks like 2.1 material now...

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.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
>