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Re: [Qemu-devel] [PATCH 1/1] migration: fix deadlock
From: |
Juan Quintela |
Subject: |
Re: [Qemu-devel] [PATCH 1/1] migration: fix deadlock |
Date: |
Wed, 30 Sep 2015 18:16:54 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux) |
"Denis V. Lunev" <address@hidden> 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 <address@hidden>
> Reported-by: Igor Redko <address@hidden>
> Tested-by: Igor Redko <address@hidden>
> CC: Anna Melekhova <address@hidden>
> CC: Juan Quintela <address@hidden>
> CC: Amit Shah <address@hidden>
> CC: Paolo Bonzini <address@hidden>
> CC: Wen Congyang <address@hidden>
Reviewed-by: Juan Quintela <address@hidden>
Appliefd to my tree.
PD, no I still don't understood how RCU gave us so many corner cases wrong.