Re: [Qemu-devel] [PATCH 1/1] migration: fix deadlock

[Amit Shah <amit.shah@redhat.com>]

There have been multiple versions of this patch on the list, can you
please annotate that this is v3 so it supersedes the earlier v2?

Also, please include a changelog in the description in patch 0 so we
know what happened between the various versions.

Thanks,

On (Mon) 28 Sep 2015 [14:41:58], Denis V. Lunev wrote:
> Release qemu global mutex before call synchronize_rcu().
> synchronize_rcu() waiting for all readers to finish their critical
> sections. There is at least one critical section in which we try
> to get QGM (critical section is in address_space_rw() and
> prepare_mmio_access() is trying to aquire QGM).
> 
> Both functions (migration_end() and migration_bitmap_extend())
> are called from main thread which is holding QGM.
> 
> Thus there is a race condition that ends up with deadlock:
> main thread     working thread
> Lock QGA                |
> |             Call KVM_EXIT_IO handler
> |                       |
> |        Open rcu reader's critical section
> Migration cleanup bh    |
> |                       |
> synchronize_rcu() is    |
> waiting for readers     |
> |            prepare_mmio_access() is waiting for QGM
>   \                   /
>          deadlock
> 
> The patch changes bitmap freeing from direct g_free after synchronize_rcu
> to free inside call_rcu.
> 
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> Reported-by: Igor Redko <redkoi@virtuozzo.com>
> Tested-by: Igor Redko <redkoi@virtuozzo.com>
> CC: Anna Melekhova <annam@virtuozzo.com>
> CC: Juan Quintela <quintela@redhat.com>
> CC: Amit Shah <amit.shah@redhat.com>
> CC: Paolo Bonzini <pbonzini@redhat.com>
> CC: Wen Congyang <wency@cn.fujitsu.com>
> ---
>  migration/ram.c | 44 +++++++++++++++++++++++++++-----------------
>  1 file changed, 27 insertions(+), 17 deletions(-)

		Amit