Experimental fork of QEMU with video encoding patches https://git.openslx.org/bwlp/qemu.git/atom/docs/aio_notify_bug.promela?h=spice_video_codecs 2017-06-07T16:22:03+00:00 2017-06-07T16:22:03+00:00 Paolo Bonzini 2017-06-06T14:46:26+00:00 urn:sha1:ac06724a715864942e2b5e28f92d5d5421f0a0b0 Developer documentation should be its own manual. As a start, move all developer-oriented files to a separate directory. Also move non-text files to their own directories: docs/config/ for QEMU -readconfig input, and docs/spin/ for formal models to be used with the SPIN model checker. Reviewed-by: Daniel P. Berrange <berrange@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> 2015-07-22T11:41:40+00:00 Paolo Bonzini 2015-07-21T14:07:52+00:00 urn:sha1:21a03d17f2edb1e63f7137d97ba355cc6f19d79f 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>