Re: [Qemu-devel] [PATCH v2 0/5] dataplane snapshot fixes

[Denis V. Lunev]

On 10/27/2015 09:41 PM, Paolo Bonzini wrote:
> On 27/10/2015 15:09, Denis V. Lunev wrote:
> > The following test
> >      while /bin/true ; do
> >          virsh snapshot-create rhel7
> >          sleep 10
> >          virsh snapshot-delete rhel7 --current
> >      done
> > with enabled iothreads on a running VM leads to a lot of troubles: hangs,
> > asserts, errors.
> >
> > Though (in general) HMP snapshot code is terrible. I think it should be
> > dropped at once and replaced with blkdev transactions code. Though is
> > could not fit to QEMU 2.5/stable at all.
> >
> > Anyway, I think that the construction like
> >      assert(aio_context_is_locked(aio_context));
> > should be widely used to ensure proper locking.
> >
> > Changes from v1:
> > - aio-context locking added
> > - comment is rewritten
> >
> > Signed-off-by: Denis V. Lunev <address@hidden>
> > CC: Stefan Hajnoczi <address@hidden>
> > CC: Paolo Bonzini <address@hidden>
> For patches 4-5:
>
> Reviewed-by: Paolo Bonzini <address@hidden>
>
> For patches 1-3 I'm not sure, because we will remove RFifoLock
> relatively soon and regular pthread recursive mutexes do not have an
> equivalent of rfifolock_is_locked.
>
> Paolo

This does not break any future.

Yes, FifoLock will go away, but aio_context_is_locked will
survive like it stays in the kernel code. We can either have
plain pthread_mutex_try_lock/unlock at first or we can
have additional stubs for linux with checks like this

(gdb)  p *(pthread_mutex_t*)0x6015a0
$3  =  {
  __data  =  {
    __lock  =  2,
    __count  =  0,
    __owner  =  12276,   <==  LWP12276  is Thread 3
    __nusers  =  1,
    __kind  =  0,        <==  non-recursive
    __spins  =  0,
    __list  =  {
      __prev  =  0x0,
      __next  =  0x0
    }
  },
  __size  =      "\002\000\000\000\000\000\000\000\364/\000\000\001",'\000'  
<repeats26  times>,
  __align  =  2
}

in debug mode. Yes, they relays on internal representation,
but they are useful.

This assert was VERY useful for me. I presume that there are
a LOT of similar places in the code with different functions
where aio_context lock was not acquired and there was no
way to ensure consistency.

Den