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21a03d17f2
event_notifier_test_and_clear must be called before processing events.
Otherwise, an aio_poll could "eat" the notification before the main
I/O thread invokes ppoll(). The main I/O thread then never wakes up.
This is an example of what could happen:
i/o thread vcpu thread worker thread
---------------------------------------------------------------------
lock_iothread
notify_me = 1
...
unlock_iothread
bh->scheduled = 1
event_notifier_set
lock_iothread
notify_me = 3
ppoll
notify_me = 1
aio_dispatch
aio_bh_poll
thread_pool_completion_bh
bh->scheduled = 1
event_notifier_set
node->io_read(node->opaque)
event_notifier_test_and_clear
ppoll
*** hang ***
"Tracing" with qemu_clock_get_ns shows pretty much the same behavior as
in the previous bug, so there are no new tricks here---just stare more
at the code until it is apparent.
One could also use a formal model, of course. The included one shows
this with three processes: notifier corresponds to a QEMU thread pool
worker, temporary_waiter to a VCPU thread that invokes aio_poll(),
waiter to the main I/O thread. I would be happy to say that the
formal model found the bug for me, but actually I wrote it after the
fact.
This patch is a bit of a big hammer. The next one optimizes it,
with help (this time for real rather than a posteriori :)) from
another, similar formal model.
Reported-by: Richard W. M. Jones <rjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Message-id: 1437487673-23740-6-git-send-email-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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|---|---|---|
| .. | ||
| qmp | ||
| specs | ||
| aio_notify.promela | ||
| aio_notify_bug.promela | ||
| atomics.txt | ||
| bitmaps.md | ||
| blkdebug.txt | ||
| blkverify.txt | ||
| bootindex.txt | ||
| ccid.txt | ||
| ich9-ehci-uhci.cfg | ||
| image-fuzzer.txt | ||
| libcacard.txt | ||
| live-block-ops.txt | ||
| memory-hotplug.txt | ||
| memory.txt | ||
| migration.txt | ||
| multi-thread-compression.txt | ||
| multiple-iothreads.txt | ||
| multiseat.txt | ||
| pci_expander_bridge.txt | ||
| q35-chipset.cfg | ||
| qapi-code-gen.txt | ||
| qdev-device-use.txt | ||
| qemupciserial.inf | ||
| rcu.txt | ||
| rdma.txt | ||
| spice-port-fqdn.txt | ||
| tracing.txt | ||
| usb-storage.txt | ||
| usb2.txt | ||
| virtio-balloon-stats.txt | ||
| vnc-ledstate-Pseudo-encoding.txt | ||
| writing-qmp-commands.txt | ||
| xbzrle.txt | ||
| xen-save-devices-state.txt | ||