s390 migration crash

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

Hi Peter's,
  Peter M pointed me to a seg in a migration test in CI; I can reproduce
it:
  * On an s390 host
  * only as part of a make check - running migration-test by itself
doesn't trigger for me.
  * It looks like it's postcopy preempt

(gdb) bt full
#0  iov_size (iov=iov@entry=0x2aa00e60670, iov_cnt=<optimized out>) at ../util/iov.c:88
        len = 13517923312037845750
        i = 17305
#1  0x000002aa004d068c in qemu_fflush (f=0x2aa00e58630) at ../migration/qemu-file.c:307
        local_error = 0x0
#2  0x000002aa004d0e04 in qemu_fflush (f=<optimized out>) at ../migration/qemu-file.c:297
#3  0x000002aa00613962 in postcopy_preempt_shutdown_file (s=s@entry=0x2aa00d1b4e0) at ../migration/ram.c:4657
#4  0x000002aa004e12b4 in migration_completion (s=0x2aa00d1b4e0) at ../migration/migration.c:3469
        ret = <optimized out>
        current_active_state = 5
        must_precopy = 0
        can_postcopy = 0
        in_postcopy = true
        pending_size = 0
        __func__ = "migration_iteration_run"
        iter_state = <optimized out>
        s = 0x2aa00d1b4e0
        thread = <optimized out>
        setup_start = <optimized out>
        thr_error = <optimized out>
        urgent = <optimized out>
#5  migration_iteration_run (s=0x2aa00d1b4e0) at ../migration/migration.c:3882
        must_precopy = 0
        can_postcopy = 0
        in_postcopy = true
        pending_size = 0
        __func__ = "migration_iteration_run"
        iter_state = <optimized out>
        s = 0x2aa00d1b4e0
        thread = <optimized out>
        setup_start = <optimized out>
        thr_error = <optimized out>
        urgent = <optimized out>
#6  migration_thread (opaque=opaque@entry=0x2aa00d1b4e0) at ../migration/migration.c:4124
        iter_state = <optimized out>
        s = 0x2aa00d1b4e0
--Type <RET> for more, q to quit, c to continue without paging--
        thread = <optimized out>
        setup_start = <optimized out>
        thr_error = <optimized out>
        urgent = <optimized out>
#7  0x000002aa00819b8c in qemu_thread_start (args=<optimized out>) at ../util/qemu-thread-posix.c:541
        __cancel_buf = 
            {__cancel_jmp_buf = {{__cancel_jmp_buf = {{__gregs = {4396782422080, 4393751543808, 4397299389454, 4396844235904, 2929182727824, 2929182933488, 4396843986792, 4397299389455, 33679382915066768, 33678512846981306}, __fpregs = {4396774031360, 8392704, 2929182933488, 0, 4396782422272, 2929172491858, 4396774031360, 1}}}, __mask_was_saved = 0}}, __pad = {0x3ffb4a77a60, 0x0, 0x0, 0x0}}
        __cancel_routine = 0x2aa00819bf0 <qemu_thread_atexit_notify>
        __not_first_call = <optimized out>
        start_routine = 0x2aa004e08f0 <migration_thread>
        arg = 0x2aa00d1b4e0
        r = <optimized out>
#8  0x000003ffb7b1e2e6 in start_thread () at /lib64/libc.so.6
#9  0x000003ffb7aafdbe in thread_start () at /lib64/libc.so.6

It looks like it's in the preempt test:

(gdb) where
#0  0x000003ffb17a0126 in __pthread_kill_implementation () from /lib64/libc.so.6
#1  0x000003ffb1750890 in raise () from /lib64/libc.so.6
#2  0x000003ffb172a340 in abort () from /lib64/libc.so.6
#3  0x000002aa0041c130 in qtest_check_status (s=<optimized out>) at ../tests/qtest/libqtest.c:194
#4  0x000003ffb1a3b5de in g_hook_list_invoke () from /lib64/libglib-2.0.so.0
#5  <signal handler called>
#6  0x000003ffb17a0126 in __pthread_kill_implementation () from /lib64/libc.so.6
#7  0x000003ffb1750890 in raise () from /lib64/libc.so.6
#8  0x000003ffb172a340 in abort () from /lib64/libc.so.6
#9  0x000002aa00420318 in qmp_fd_receive (fd=<optimized out>) at ../tests/qtest/libqmp.c:80
#10 0x000002aa0041d5ee in qtest_qmp_receive_dict (s=0x2aa01eb2700) at ../tests/qtest/libqtest.c:713
#11 qtest_qmp_receive (s=0x2aa01eb2700) at ../tests/qtest/libqtest.c:701
#12 qtest_vqmp (s=s@entry=0x2aa01eb2700, fmt=fmt@entry=0x2aa00487100 "{ 'execute': 'query-migrate' }", ap=ap@entry=0x3ffc247cc68)
    at ../tests/qtest/libqtest.c:765
#13 0x000002aa00413f1e in wait_command (who=who@entry=0x2aa01eb2700, command=command@entry=0x2aa00487100 "{ 'execute': 'query-migrate' }")
    at ../tests/qtest/migration-helpers.c:73
#14 0x000002aa00414078 in migrate_query (who=who@entry=0x2aa01eb2700) at ../tests/qtest/migration-helpers.c:139
#15 migrate_query_status (who=who@entry=0x2aa01eb2700) at ../tests/qtest/migration-helpers.c:161
#16 0x000002aa00414480 in check_migration_status (ungoals=0x0, goal=0x2aa00495c7e "completed", who=0x2aa01eb2700) at ../tests/qtest/migration-helpers.c:177
#17 wait_for_migration_status (who=0x2aa01eb2700, goal=<optimized out>, ungoals=0x0) at ../tests/qtest/migration-helpers.c:202
#18 0x000002aa0041300e in migrate_postcopy_complete (from=from@entry=0x2aa01eb2700, to=to@entry=0x2aa01eb3000, args=args@entry=0x3ffc247cf48)
    at ../tests/qtest/migration-test.c:1137
#19 0x000002aa004131a4 in test_postcopy_common (args=0x3ffc247cf48) at ../tests/qtest/migration-test.c:1162
#20 test_postcopy_preempt () at ../tests/qtest/migration-test.c:1178

Looking at the iov and file it's garbage; so it makes me think this is
something like a flush on a closed file.

Dave

-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK