Re: [Qemu-devel] [BUG?] aio_get_linux_aio: Assertion `ctx->linux_aio' failed

[Nishanth Aravamudan]

On 17.07.2018 [13:25:53 -0400], Farhan Ali wrote:
> Hi,
> 
> I am seeing some strange QEMU assertion failures for qemu on s390x,
> which prevents a guest from starting.
> 
> Git bisecting points to the following commit as the source of the error.
> 
> commit ed6e2161715c527330f936d44af4c547f25f687e
> Author: Nishanth Aravamudan <address@hidden>
> Date:   Fri Jun 22 12:37:00 2018 -0700
> 
>     linux-aio: properly bubble up errors from initialization
> 
>     laio_init() can fail for a couple of reasons, which will lead to a NULL
>     pointer dereference in laio_attach_aio_context().
> 
>     To solve this, add a aio_setup_linux_aio() function which is called
>     early in raw_open_common. If this fails, propagate the error up. The
>     signature of aio_get_linux_aio() was not modified, because it seems
>     preferable to return the actual errno from the possible failing
>     initialization calls.
> 
>     Additionally, when the AioContext changes, we need to associate a
>     LinuxAioState with the new AioContext. Use the bdrv_attach_aio_context
>     callback and call the new aio_setup_linux_aio(), which will allocate a
>     new AioContext if needed, and return errors on failures. If it fails for
>     any reason, fallback to threaded AIO with an error message, as the
>     device is already in-use by the guest.
> 
>     Add an assert that aio_get_linux_aio() cannot return NULL.
> 
>     Signed-off-by: Nishanth Aravamudan <address@hidden>
>     Message-id: address@hidden
>     Signed-off-by: Stefan Hajnoczi <address@hidden>
> 
> 
> Not sure what is causing this assertion to fail. Here is the qemu command
> line of the guest, from qemu log, which throws this error:
> 
> 
> LC_ALL=C PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin
> QEMU_AUDIO_DRV=none /usr/local/bin/qemu-system-s390x -name
> guest=rt_vm1,debug-threads=on -S -object 
> secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-21-rt_vm1/master-key.aes
> -machine s390-ccw-virtio-2.12,accel=kvm,usb=off,dump-guest-core=off -m 1024
> -realtime mlock=off -smp 4,sockets=4,cores=1,threads=1 -object
> iothread,id=iothread1 -uuid 0cde16cd-091d-41bd-9ac2-5243df5c9a0d -display
> none -no-user-config -nodefaults -chardev
> socket,id=charmonitor,fd=28,server,nowait -mon
> chardev=charmonitor,id=monitor,mode=control -rtc base=utc -no-shutdown -boot
> strict=on -drive 
> file=/dev/mapper/360050763998b0883980000002a000031,format=raw,if=none,id=drive-virtio-disk0,cache=none,aio=native
> -device 
> virtio-blk-ccw,iothread=iothread1,scsi=off,devno=fe.0.0001,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1,write-cache=on
> -netdev tap,fd=30,id=hostnet0,vhost=on,vhostfd=31 -device
> virtio-net-ccw,netdev=hostnet0,id=net0,mac=02:3a:c8:67:95:84,devno=fe.0.0000
> -netdev tap,fd=32,id=hostnet1,vhost=on,vhostfd=33 -device
> virtio-net-ccw,netdev=hostnet1,id=net1,mac=52:54:00:2a:e5:08,devno=fe.0.0002
> -chardev pty,id=charconsole0 -device
> sclpconsole,chardev=charconsole0,id=console0 -device
> virtio-balloon-ccw,id=balloon0,devno=fe.3.ffba -sandbox
> on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny -msg
> timestamp=on
> 
> 
> 
> 2018-07-17 15:48:42.252+0000: Domain id=21 is tainted: high-privileges
> 2018-07-17T15:48:42.279380Z qemu-system-s390x: -chardev pty,id=charconsole0:
> char device redirected to /dev/pts/3 (label charconsole0)
> qemu-system-s390x: util/async.c:339: aio_get_linux_aio: Assertion
> `ctx->linux_aio' failed.
> 2018-07-17 15:48:43.309+0000: shutting down, reason=failed
> 
> 
> Any help debugging this would be greatly appreciated.

iiuc, this possibly implies AIO was not actually used previously on this
guest (it might have silently been falling back to threaded IO?). I
don't have access to s390x, but would it be possible to run qemu under
gdb and see if aio_setup_linux_aio is being called at all (I think it
might not be, but I'm not sure why), and if so, if it's for the context
in question?

If it's not being called first, could you see what callpath is calling
aio_get_linux_aio when this assertion trips?

Thanks!
-Nish