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[Qemu-devel] [PATCH v2 0/2] migration: fix deadlock


From: Denis V. Lunev
Subject: [Qemu-devel] [PATCH v2 0/2] migration: fix deadlock
Date: Fri, 25 Sep 2015 12:09:38 +0300

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

Patches here are quick and dirty, compile-tested only to validate the
architectual approach.

Igor, Anna, can you pls start your tests with these patches instead of your
original one. Thank you.

Signed-off-by: Denis V. Lunev <address@hidden>
CC: Igor Redko <address@hidden>
CC: Anna Melekhova <address@hidden>
CC: Juan Quintela <address@hidden>
CC: Amit Shah <address@hidden>

Denis V. Lunev (2):
  migration: bitmap_set is unnecessary as bitmap_new uses g_try_malloc0
  migration: fix deadlock

 migration/ram.c | 45 ++++++++++++++++++++++++++++-----------------
 1 file changed, 28 insertions(+), 17 deletions(-)

-- 
2.1.4




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