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

[Igor Redko]

On Пн., 2015-09-28 at 13:55 +0300, Igor Redko wrote:
> On Пт., 2015-09-25 at 17:46 +0800, Wen Congyang wrote:
> > On 09/25/2015 05:09 PM, 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
> > > 
> > > 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.
> > 
> > Can you give me the backtrace of the working thread?
> > 
> > I think it is very bad to wait some lock in rcu reader's cirtical section.
> 
> #0  __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135
> #1  0x00007f1ef113ccfd in __GI___pthread_mutex_lock (mutex=0x7f1ef4145ce0 
> <qemu_global_mutex>) at ../nptl/pthread_mutex_lock.c:80
> #2  0x00007f1ef3c36546 in qemu_mutex_lock (mutex=0x7f1ef4145ce0 
> <qemu_global_mutex>) at util/qemu-thread-posix.c:73
> #3  0x00007f1ef387ff46 in qemu_mutex_lock_iothread () at 
> /home/user/my_qemu/qemu/cpus.c:1170
> #4  0x00007f1ef38514a2 in prepare_mmio_access (mr=0x7f1ef612f200) at 
> /home/user/my_qemu/qemu/exec.c:2390
> #5  0x00007f1ef385157e in address_space_rw (as=0x7f1ef40ec940 
> <address_space_io>, addr=49402, attrs=..., buf=0x7f1ef3f97000 "\001", len=1, 
> is_write=true)
>     at /home/user/my_qemu/qemu/exec.c:2425
> #6  0x00007f1ef3897c53 in kvm_handle_io (port=49402, attrs=..., 
> data=0x7f1ef3f97000, direction=1, size=1, count=1) at 
> /home/user/my_qemu/qemu/kvm-all.c:1680
> #7  0x00007f1ef3898144 in kvm_cpu_exec (cpu=0x7f1ef5010fc0) at 
> /home/user/my_qemu/qemu/kvm-all.c:1849
> #8  0x00007f1ef387fa91 in qemu_kvm_cpu_thread_fn (arg=0x7f1ef5010fc0) at 
> /home/user/my_qemu/qemu/cpus.c:979
> #9  0x00007f1ef113a6aa in start_thread (arg=0x7f1eef0b9700) at 
> pthread_create.c:333
> #10 0x00007f1ef0e6feed in clone () at 
> ../sysdeps/unix/sysv/linux/x86_64/clone.S:109

Backtrace of the main thread:

#0  syscall () at ../sysdeps/unix/sysv/linux/x86_64/syscall.S:38
#1  0x00007f6ade8e9a17 in futex_wait (ev=0x7f6adf26e120 <rcu_gp_event>, 
val=4294967295) at util/qemu-thread-posix.c:301
#2  0x00007f6ade8e9b2d in qemu_event_wait (ev=0x7f6adf26e120 <rcu_gp_event>) at 
util/qemu-thread-posix.c:399
#3  0x00007f6ade8fd7ec in wait_for_readers () at util/rcu.c:120
#4  0x00007f6ade8fd875 in synchronize_rcu () at util/rcu.c:149
#5  0x00007f6ade5640fd in migration_end () at 
/home/user/my_qemu/qemu/migration/ram.c:1036
#6  0x00007f6ade564194 in ram_migration_cancel (opaque=0x0) at 
/home/user/my_qemu/qemu/migration/ram.c:1054
#7  0x00007f6ade567d5a in qemu_savevm_state_cancel () at 
/home/user/my_qemu/qemu/migration/savevm.c:915
#8  0x00007f6ade7b4bdf in migrate_fd_cleanup (opaque=0x7f6aded8fd40 
<current_migration>) at migration/migration.c:582
#9  0x00007f6ade804d15 in aio_bh_poll (ctx=0x7f6adf895e50) at async.c:87
#10 0x00007f6ade814dcb in aio_dispatch (ctx=0x7f6adf895e50) at aio-posix.c:135
#11 0x00007f6ade8050b5 in aio_ctx_dispatch (source=0x7f6adf895e50, 
callback=0x0, user_data=0x0) at async.c:226
#12 0x00007f6adc9a3c3d in g_main_context_dispatch () from 
/lib/x86_64-linux-gnu/libglib-2.0.so.0
#13 0x00007f6ade813274 in glib_pollfds_poll () at main-loop.c:208
#14 0x00007f6ade813351 in os_host_main_loop_wait (timeout=422420000) at 
main-loop.c:253
#15 0x00007f6ade813410 in main_loop_wait (nonblocking=0) at main-loop.c:502
#16 0x00007f6ade64ae6a in main_loop () at vl.c:1902
#17 0x00007f6ade652c32 in main (argc=70, argv=0x7ffcc3b674e8, 
envp=0x7ffcc3b67720) at vl.c:4653

> 
> > 
> > > 
> > > 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(-)
> > > 
> > 
>