[PATCH v2 3/7] Fixed SVM hang when do failover before PVM crash

[Rao, Lei]

From: "Rao, Lei" <lei.rao@intel.com>

This patch fixed as follows:
    Thread 1 (Thread 0x7f34ee738d80 (LWP 11212)):
    #0 __pthread_clockjoin_ex (threadid=139847152957184, 
thread_return=0x7f30b1febf30, clockid=<optimized out>, abstime=<optimized out>, 
block=<optimized out>) at pthread_join_common.c:145
    #1 0x0000563401998e36 in qemu_thread_join (thread=0x563402d66610) at 
util/qemu-thread-posix.c:587
    #2 0x00005634017a79fa in process_incoming_migration_co (opaque=0x0) at 
migration/migration.c:502
    #3 0x00005634019b59c9 in coroutine_trampoline (i0=63395504, i1=22068) at 
util/coroutine-ucontext.c:115
    #4 0x00007f34ef860660 in ?? () at 
../sysdeps/unix/sysv/linux/x86_64/__start_context.S:91 from 
/lib/x86_64-linux-gnu/libc.so.6
    #5 0x00007f30b21ee730 in ?? ()
    #6 0x0000000000000000 in ?? ()

    Thread 13 (Thread 0x7f30b3dff700 (LWP 11747)):
    #0  __lll_lock_wait (futex=futex@entry=0x56340218ffa0 <qemu_global_mutex>, 
private=0) at lowlevellock.c:52
    #1  0x00007f34efa000a3 in _GI__pthread_mutex_lock (mutex=0x56340218ffa0 
<qemu_global_mutex>) at ../nptl/pthread_mutex_lock.c:80
    #2  0x0000563401997f99 in qemu_mutex_lock_impl (mutex=0x56340218ffa0 
<qemu_global_mutex>, file=0x563401b7a80e "migration/colo.c", line=806) at 
util/qemu-thread-posix.c:78
    #3  0x0000563401407144 in qemu_mutex_lock_iothread_impl 
(file=0x563401b7a80e "migration/colo.c", line=806) at 
/home/workspace/colo-qemu/cpus.c:1899
    #4  0x00005634017ba8e8 in colo_process_incoming_thread 
(opaque=0x563402d664c0) at migration/colo.c:806
    #5  0x0000563401998b72 in qemu_thread_start (args=0x5634039f8370) at 
util/qemu-thread-posix.c:519
    #6  0x00007f34ef9fd609 in start_thread (arg=<optimized out>) at 
pthread_create.c:477
    #7  0x00007f34ef924293 in clone () at 
../sysdeps/unix/sysv/linux/x86_64/clone.S:95

    The QEMU main thread is holding the lock:
    (gdb) p qemu_global_mutex
    $1 = {lock = {_data = {lock = 2, __count = 0, __owner = 11212, __nusers = 
9, __kind = 0, __spins = 0, __elision = 0, __list = {_prev = 0x0, __next = 
0x0}},
     __size = "\002\000\000\000\000\000\000\000\314+\000\000\t", '\000' 
<repeats 26 times>, __align = 2}, file = 0x563401c07e4b "util/main-loop.c", 
line = 240,
    initialized = true}

>From the call trace, we can see it is a deadlock bug. and the QEMU main thread 
>holds the global mutex to wait until the COLO thread ends. and the colo thread
wants to acquire the global mutex, which will cause a deadlock. So, we should 
release the qemu_global_mutex before waiting colo thread ends.

Signed-off-by: Lei Rao <lei.rao@intel.com>
Reviewed-by: Li Zhijian <lizhijian@cn.fujitsu.com>
---
 migration/migration.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/migration/migration.c b/migration/migration.c
index e5b38a492f..811097c423 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -587,8 +587,10 @@ static void process_incoming_migration_co(void *opaque)
         mis->have_colo_incoming_thread = true;
         qemu_coroutine_yield();
 
+        qemu_mutex_unlock_iothread();
         /* Wait checkpoint incoming thread exit before free resource */
         qemu_thread_join(&mis->colo_incoming_thread);
+        qemu_mutex_lock_iothread();
         /* We hold the global iothread lock, so it is safe here */
         colo_release_ram_cache();
     }
-- 
2.30.2