[Qemu-devel] qemu_futex_wait() lockups in ARM64: 2 possible issues

[Rafael David Tinoco]

Paolo,

While debugging hungs in ARM64 while doing a simple:

qemu-img convert -f qcow2 -O qcow2 file.qcow2 output.qcow2

I might have found 2 issues which I'd like you to review, if possible.

ISSUE #1
========

I've caught the following stack trace after an HUNG in qemu-img convert:

(gdb) bt
#0 syscall ()
#1 0x0000aaaaaabd41cc in qemu_futex_wait
#2 qemu_event_wait (ev=ev@entry=0xaaaaaac86ce8 <rcu_call_ready_event>)
#3 0x0000aaaaaabed05c in call_rcu_thread
#4 0x0000aaaaaabd34c8 in qemu_thread_start
#5 0x0000ffffbf25c880 in start_thread
#6 0x0000ffffbf1b6b9c in thread_start ()

(gdb) print rcu_call_ready_event
$4 = {value = 4294967295, initialized = true}

value INT_MAX (4294967295) seems WRONG for qemu_futex_wait():

- EV_BUSY, being -1, and passed as an argument qemu_futex_wait(void *,
unsigned), is a two's complement, making argument into a INT_MAX when
that's not what is expected (unless I missed something).

*** If that is the case, unsure if you, Paolo, prefer declaring
*(QemuEvent)->value as an integer or changing EV_BUSY to "2" would okay
here ***

BUG: description:
https://bugs.launchpad.net/qemu/+bug/1805256/comments/15

========
ISSUE #2
========

I found this when debugging lockups while in futex() in a specific ARM64
server - https://bugs.launchpad.net/qemu/+bug/1805256 - which I'm still
investigating.

After fixing the issue above, I'm still getting stuck into:

qemu_event_wait() -> qemu_futex_wait()

***
As if qemu_event_set() has ran before qemu_futex_wait() ever started running
***

The Other threads are waiting for poll() on a PIPE coming from this
stuck thread (thread #1), and in sigwait():

(gdb) thread 1
...
(gdb) bt
#0  0x0000ffffbf1ad81c in __GI_ppoll
#1  0x0000aaaaaabcf73c in ppoll
#2  qemu_poll_ns
#3  0x0000aaaaaabd0764 in os_host_main_loop_wait
#4  main_loop_wait
...

(gdb) thread 2
...
(gdb) bt
#0 syscall ()
#1 0x0000aaaaaabd41cc in qemu_futex_wait
#2 qemu_event_wait (ev=ev@entry=0xaaaaaac86ce8 <rcu_call_ready_event>)
#3 0x0000aaaaaabed05c in call_rcu_thread
#4 0x0000aaaaaabd34c8 in qemu_thread_start
#5 0x0000ffffbf25c880 in start_thread
#6 0x0000ffffbf1b6b9c in thread_start ()

(gdb) thread 3
...
(gdb) bt
#0  0x0000ffffbf11aa20 in __GI___sigtimedwait
#1  0x0000ffffbf2671b4 in __sigwait
#2  0x0000aaaaaabd1ddc in sigwait_compat
#3  0x0000aaaaaabd34c8 in qemu_thread_start
#4  0x0000ffffbf25c880 in start_thread
#5  0x0000ffffbf1b6b9c in thread_start

QUESTION:

- Should qemu_event_set() check return code from
qemu_futex_wake()->qemu_futex()->syscall() in order to know if ANY
waiter was ever woken up ? Maybe even loop until at least 1 is awaken ?

Tks in advance,

Rafael D. Tinoco