Re: [Qemu-devel] Regression from 2.8: stuck in bdrv_drain()

[Fam Zheng]

On Wed, 04/12 18:22, Jeff Cody wrote:
> On Wed, Apr 12, 2017 at 05:38:17PM -0400, John Snow wrote:
> > 
> > 
> > On 04/12/2017 04:46 PM, Jeff Cody wrote:
> > > 
> > > This occurs on v2.9.0-rc4, but not on v2.8.0.
> > > 
> > > When running QEMU with an iothread, and then performing a block-mirror, if
> > > we do a system-reset after the BLOCK_JOB_READY event has emitted, qemu
> > > becomes deadlocked.
> > > 
> > > The block job is not paused, nor cancelled, so we are stuck in the while
> > > loop in block_job_detach_aio_context:
> > > 
> > > static void block_job_detach_aio_context(void *opaque)
> > > {
> > >     BlockJob *job = opaque;
> > > 
> > >     /* In case the job terminates during aio_poll()... */
> > >     block_job_ref(job);
> > > 
> > >     block_job_pause(job);
> > > 
> > >     while (!job->paused && !job->completed) {
> > >         block_job_drain(job);
> > >     }
> > > 
> > 
> > Looks like when block_job_drain calls block_job_enter from this context
> > (the main thread, since we're trying to do a system_reset...), we cannot
> > enter the coroutine because it's the wrong context, so we schedule an
> > entry instead with
> > 
> > aio_co_schedule(ctx, co);
> > 
> > But that entry never happens, so the job never wakes up and we never
> > make enough progress in the coroutine to gracefully pause, so we wedge here.
> > 
> 
> 
> John Snow and I debugged this some over IRC.  Here is a summary:
> 
> Simply put, with iothreads the aio context is different.  When
> block_job_detach_aio_context() is called from the main thread via the system
> reset (from main_loop_should_exit()), it calls block_job_drain() in a while
> loop, with job->busy and job->completed as exit conditions.
> 
> block_job_drain() attempts to enter the coroutine (thus allowing job->busy
> or job->completed to change).  However, since the aio context is different
> with iothreads, we schedule the coroutine entry rather than directly
> entering it.
> 
> This means the job coroutine is never going to be re-entered, because we are
> waiting for it to complete in a while loop from the main thread, which is
> blocking the qemu timers which would run the scheduled coroutine... hence,
> we become stuck.

John and I confirmed that this can be fixed by this pending patch:

[PATCH for-2.9 4/5] block: Drain BH in bdrv_drained_begin

https://lists.gnu.org/archive/html/qemu-devel/2017-04/msg01018.html

It didn't make it into 2.9-rc4 because of limited time. :(

Looks like there is no -rc5, we'll have to document this as a known issue.
Users should "block-job-complete/cancel" as soon as possible to avoid such a
hang.

Fam