Re: [PULL 09/14] qmp: Move dispatcher to a coroutine

[Alex Bennée]

Kevin Wolf <kwolf@redhat.com> writes:

> Am 12.10.2020 um 13:53 hat Philippe Mathieu-Daudé geschrieben:
>> On 10/12/20 1:25 PM, Kevin Wolf wrote:
>> > Am 12.10.2020 um 12:47 hat Alex Bennée geschrieben:
>> > > 
>> > > Markus Armbruster <armbru@redhat.com> writes:
>> > > 
>> > > > From: Kevin Wolf <kwolf@redhat.com>
>> > > > 
>> > > > This moves the QMP dispatcher to a coroutine and runs all QMP command
>> > > > handlers that declare 'coroutine': true in coroutine context so they
>> > > > can avoid blocking the main loop while doing I/O or waiting for other
>> > > > events.
>> > > 
>> > > This subtly changes the replay behaviour leading to a hang in:
>> > > 
>> > >    10:55:18 [alex.bennee@hackbox2:~/l/q/b/bisect] (625581c2…)|✚1(+1/-1) 
>> > > + ./tests/venv/bin/avocado run 
>> > > tests/acceptance/replay_kernel.py:ReplayKernel.test_arm_virt
>> > >    Fetching asset from 
>> > > tests/acceptance/replay_kernel.py:ReplayKernel.test_arm_virt
>> > >    JOB ID     : ec11fd2544f06e6c0d421f16afa757b49f7ed734
>> > >    JOB LOG    : 
>> > > /home/alex.bennee/avocado/job-results/job-2020-10-12T11.40-ec11fd2/job.log
>> > >     (1/1) tests/acceptance/replay_kernel.py:ReplayKernel.test_arm_virt: 
>> > > ERROR: Could not perform graceful shutdown (26.27 s)
>> > >    RESULTS    : PASS 0 | ERROR 1 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 
>> > > 0 | CANCEL 0
>> > >    JOB TIME   : 27.77 s
>> > > 
>> > > Looking at the log:
>> > > 
>> > >    2020-10-12 11:40:31,426 __init__         L0085 DEBUG| [    3.887411] 
>> > > rtc-pl031 9010000.pl031: setting system clock to 2020-10-12 10:40:31 UTC 
>> > > (1602499231)
>> > >    2020-10-12 11:40:31,428 __init__         L0085 DEBUG| [    3.887431] 
>> > > sr_init: No PMIC hook to init smartreflex
>> > >    2020-10-12 11:40:31,447 __init__         L0085 DEBUG| [    3.897193] 
>> > > uart-pl011 9000000.pl011: no DMA platform data
>> > >    2020-10-12 11:40:31,460 __init__         L0085 DEBUG| [    3.897242] 
>> > > md: Waiting for all devices to be available before autodetect
>> > >    2020-10-12 11:40:31,462 __init__         L0085 DEBUG| [    3.897259] 
>> > > md: If you don't use raid, use raid=noautodetect
>> > >    2020-10-12 11:40:31,475 __init__         L0085 DEBUG| [    3.897819] 
>> > > md: Autodetecting RAID arrays.
>> > >    2020-10-12 11:40:31,476 __init__         L0085 DEBUG| [    3.897832] 
>> > > md: autorun ...
>> > >    2020-10-12 11:40:31,477 __init__         L0085 DEBUG| [    3.897842] 
>> > > md: ... autorun DONE.
>> > >    2020-10-12 11:40:31,483 __init__         L0085 DEBUG| [    3.897962] 
>> > > VFS: Cannot open root device "(null)" or unknown-block(0,0): error -6
>> > >    2020-10-12 11:40:31,483 qmp              L0245 DEBUG| >>> {'execute': 
>> > > 'quit'}
>> > >    2020-10-12 11:40:31,495 qmp              L0145 DEBUG| <<< 
>> > > {'timestamp': {'seconds': 1602499231, 'microseconds': 493379}, 'event': 
>> > > 'SHUTDOWN', 'data': {'guest': True, 'reason': 'guest-reset'}}
>> > >    2020-10-12 11:40:31,733 machine          L0325 WARNI| qemu received 
>> > > signal 6; command: "./qemu-system-arm -display none -vga none -chardev 
>> > > socket,id=mon,path=/var/tmp/tmpzls53khe/qemu-8487-monitor.sock -mon 
>> > > chardev=mon,mode=control -machine virt -chardev 
>> > > socket,id=console,path=/var/tmp/tmpzls53khe/qemu-8487-console.sock,server,nowait
>> > >  -serial chardev:console -icount 
>> > > shift=1,rr=record,rrfile=/var/tmp/avocado_n00stdrf/avocado_job_aw60qdul/1-tests_acceptance_replay_kernel.py_ReplayKernel.test_arm_virt/replay.bin
>> > >  -kernel 
>> > > /home/alex.bennee/avocado/data/cache/by_location/62750ce9e069e69e6a7ff04ff54c382ee660b92a/vmlinuz
>> > >  -append printk.time=1 panic=-1 console=ttyAMA0 -net none -no-reboot"
>> > 
>> > This looks like a crash (SIGABRT) rather than a hang. Do you have a
>> > stack trace for the crashed process?
>> 
>> No crash, exit(0):
>
> Why does the log say "qemu received signal 6" then?
>
>> VFS: Cannot open root device "(null)" or unknown-block(0,0): error -6
>
> Alex has this error in the logs before this commit, so I assume this is
> expected. All of the following is then probably expected, too, because
> it follows directly from this error:
>
>> Please append a correct "root=" boot option; here are the available
>> partitions:
>> Kernel panic - not syncing: VFS: Unable to mount root fs on
>> unknown-block(0,0)
>> CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.18.16-300.fc29.armv7hl #1
>> Hardware name: Generic DT based system
>> [<c0313f7c>] (unwind_backtrace) from [<c030dc64>] (show_stack+0x20/0x24)
>> [<c030dc64>] (show_stack) from [<c0b50ec4>] (dump_stack+0x88/0xa8)
>> [<c0b50ec4>] (dump_stack) from [<c03592f8>] (panic+0xd4/0x26c)
>> [<c03592f8>] (panic) from [<c110183c>] (mount_block_root+0x250/0x2ec)
>> [<c110183c>] (mount_block_root) from [<c1101950>] (mount_root+0x78/0x90)
>> [<c1101950>] (mount_root) from [<c1101ac4>] (prepare_namespace+0x15c/0x19c)
>> [<c1101ac4>] (prepare_namespace) from [<c11012e8>]
>> (kernel_init_freeable+0x2c0/0x370)
>> [<c11012e8>] (kernel_init_freeable) from [<c0b63914>]
>> (kernel_init+0x18/0x128)
>> [<c0b63914>] (kernel_init) from [<c03010e8>] (ret_from_fork+0x14/0x2c)
>> Exception stack(0xc790bfb0 to 0xc790bff8)
>> bfa0:                                     00000000 00000000 00000000
>> 00000000
>> bfc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000
>> 00000000
>> bfe0: 00000000 00000000 00000000 00000000 00000013 00000000
>> 
>> -> PSCI call
>>    -> QEMU_PSCI_0_2_FN_SYSTEM_RESET
>>       -> SHUTDOWN_CAUSE_GUEST_RESET
>>          -> exit(0)

Yes - the test is recording the kernel up until the point it gives up.

> Hm... So you're saying that the test sends a 'quit' QMP command, but
> before it could be processed, the guest causes QEMU to exit, so the test
> will never receive a reply to its request?
>
> If commit 9ce44e2ce2 changes anything about this, it seems to me that it
> would be that more QMP commands are processed during monitor_cleanup()
> because it doesn't just delete the dispatcher BH, but waits until it's
> not busy any more.
>
> Looking at this code again, however, the order in monitor_cleanup() is
> probably wrong. We should first shut down the dispatcher (which may
> still be using Monitor objects) and then destroy the monitors. This
> could possibly explain a crash, but probably not wrong results with a
> successful shutdown.

I suspect this is a race between QEMU shutting down because the guest
shut it down and the acceptance test trying to shut things down via QMP.
I think the proper fix is either to:

  - s/panic=-1/panic=0/ in the command line (and probably drop --no-reboot)

which would let the acceptance test cleanly shutdown via QMP. Or:

  - modify the test to declare that qemu will shutdown itself and
    therefor no "quit" needs to be sent

WDYT?

-- 
Alex Bennée