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

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, Jul 8, 2014 at 9:07 PM, Christian Borntraeger
<borntraeger@de.ibm.com> wrote:
> 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?

Could affect performance because of the extra poll/release/acquire but
a clean solution for broken iothread_run().

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

Good for 2.1