Re: [Qemu-devel] [PATCH for-2.4 0/2] AioContext: fix deadlock after aio_context_acquire() race

[Stefan Hajnoczi]

On Tue, Jul 28, 2015 at 10:02:26AM +0200, Cornelia Huck wrote:
> On Tue, 28 Jul 2015 09:07:00 +0200
> Cornelia Huck <address@hidden> wrote:
> 
> > On Mon, 27 Jul 2015 17:33:37 +0100
> > Stefan Hajnoczi <address@hidden> wrote:
> > 
> > > See Patch 2 for details on the deadlock after two aio_context_acquire() 
> > > calls
> > > race.  This caused dataplane to hang on startup.
> > > 
> > > Patch 1 is a memory leak fix for AioContext that's needed by Patch 2.
> > > 
> > > Stefan Hajnoczi (2):
> > >   AioContext: avoid leaking BHs on cleanup
> > >   AioContext: force event loop iteration using BH
> > > 
> > >  async.c             | 29 +++++++++++++++++++++++++++--
> > >  include/block/aio.h |  3 +++
> > >  2 files changed, 30 insertions(+), 2 deletions(-)
> > > 
> > 
> > Just gave this a try: The stripped-down guest that hangs during startup
> > on master is working fine with these patches applied, and my full setup
> > works as well.
> > 
> > So,
> > 
> > Tested-by: Cornelia Huck <address@hidden>
> 
> Uh-oh, spoke too soon. It starts, but when I try a virsh managedsave, I
> get
> 
> qemu-system-s390x: /data/git/yyy/qemu/async.c:242: aio_ctx_finalize: 
> Assertion `ctx->first_bh->deleted' failed.

Please pretty-print ctx->first_bh in gdb.  In particular, which function
is ctx->first_bh->cb pointing to?

I tried reproducing with qemu-system-x86_64 and a RHEL 7 guest but
couldn't trigger the assertion failure.

This assertion means that there is an *existing* QEMUBH memory leak.  It
is not introduced by this patch series.  If we run out of time for QEMU
2.4, it would be acceptable to replace the assertion with:

  /* TODO track down leaked BHs and turn this into an assertion */
  if (ctx->first_bh->deleted) {
      g_free(ctx->first_bh);
  }

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