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qemu_laio_process_completions() wraps its body in defer_call_begin /
defer_call_end. Inside the section, completion callbacks wake coroutines
that queue new aiocbs; laio_do_submit() defers laio_deferred_fn. At the
bottom of qemu_laio_process_completions() the defer_call_end() fires
laio_deferred_fn, which calls ioq_submit(), closing the cycle:
ioq_submit
-> io_submit(2) // some sync completions
-> qemu_laio_process_completions // defer_call_begin
-> aio_co_wake // resumes coroutine
-> laio_do_submit
-> defer_call(laio_deferred_fn, s) // enqueued
-> defer_call_end // nesting drops to 0
-> laio_deferred_fn
-> ioq_submit // +1 stack frame, loop
When io_submit(2) returns asynchronously (O_DIRECT) the cycle
terminates in one extra frame: the fresh aiocb is still in flight, no
completion is drained, no coroutine wakes, no new submission queues.
When submissions complete synchronously (non-O_DIRECT, or per-descriptor
drivers such as vmdk) each level enqueues more work for the next
defer_call_end() to drain, so recursion grows without bound and QEMU
crashes with SIGSEGV on the thread guard page.
The cycle was closed by two performance commits, each correct in
isolation:
076682885d ("block/linux-aio: convert to blk_io_plug_call() API")
-- introduced laio_deferred_fn and wired
laio_do_submit -> defer_call(laio_deferred_fn, s).
84d61e5f36 ("virtio: use defer_call() in virtio_irqfd_notify()")
-- added defer_call_begin/end around qemu_laio_process_completions
so virtio-irqfd notifications batch across a completion pass.
The supported aio=native + cache=none pairing keeps submissions
asynchronous, so the cycle stays bounded; nothing in the code enforces
that contract. Observed in production as a SIGSEGV during a backup job
configured with --cached + aio=native; reproducible on upstream with
qemu-io against vmdk.
Cap ioq_submit() recursion with a counter on LaioQueue, which is only
accessed from the AioContext home thread. On overflow, return without
submitting. The pending work is drained by s->completion_bh, which
qemu_laio_process_completions() has already scheduled on entry -- no
work is lost; one event-loop round-trip of latency is paid only when
the bound is hit, which cannot happen on a supported configuration.
Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Hanna Reitz <hreitz@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20260520142503.251959-2-den@openvz.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
=========== QEMU README =========== QEMU is a generic and open source machine & userspace emulator and virtualizer. QEMU is capable of emulating a complete machine in software without any need for hardware virtualization support. By using dynamic translation, it achieves very good performance. QEMU can also integrate with the Xen and KVM hypervisors to provide emulated hardware while allowing the hypervisor to manage the CPU. With hypervisor support, QEMU can achieve near native performance for CPUs. When QEMU emulates CPUs directly it is capable of running operating systems made for one machine (e.g. an ARMv7 board) on a different machine (e.g. an x86_64 PC board). QEMU is also capable of providing userspace API virtualization for Linux and BSD kernel interfaces. This allows binaries compiled against one architecture ABI (e.g. the Linux PPC64 ABI) to be run on a host using a different architecture ABI (e.g. the Linux x86_64 ABI). This does not involve any hardware emulation, simply CPU and syscall emulation. QEMU aims to fit into a variety of use cases. It can be invoked directly by users wishing to have full control over its behaviour and settings. It also aims to facilitate integration into higher level management layers, by providing a stable command line interface and monitor API. It is commonly invoked indirectly via the libvirt library when using open source applications such as oVirt, OpenStack and virt-manager. QEMU as a whole is released under the GNU General Public License, version 2. For full licensing details, consult the LICENSE file. Documentation ============= Documentation can be found hosted online at `<https://www.qemu.org/documentation/>`_. The documentation for the current development version that is available at `<https://www.qemu.org/docs/master/>`_ is generated from the ``docs/`` folder in the source tree, and is built by `Sphinx <https://www.sphinx-doc.org/en/master/>`_. Building ======== QEMU is multi-platform software intended to be buildable on all modern Linux platforms, OS-X, Win32 (via the Mingw64 toolchain) and a variety of other UNIX targets. The simple steps to build QEMU are: .. code-block:: shell mkdir build cd build ../configure make Additional information can also be found online via the QEMU website: * `<https://wiki.qemu.org/Hosts/Linux>`_ * `<https://wiki.qemu.org/Hosts/Mac>`_ * `<https://wiki.qemu.org/Hosts/W32>`_ Submitting patches ================== The QEMU source code is maintained under the GIT version control system. .. code-block:: shell git clone https://gitlab.com/qemu-project/qemu.git When submitting patches, one common approach is to use 'git format-patch' and/or 'git send-email' to format & send the mail to the qemu-devel@nongnu.org mailing list. All patches submitted must contain a 'Signed-off-by' line from the author. Patches should follow the guidelines set out in the `style section <https://www.qemu.org/docs/master/devel/style.html>`_ of the Developers Guide. Additional information on submitting patches can be found online via the QEMU website: * `<https://wiki.qemu.org/Contribute/SubmitAPatch>`_ * `<https://wiki.qemu.org/Contribute/TrivialPatches>`_ The QEMU website is also maintained under source control. .. code-block:: shell git clone https://gitlab.com/qemu-project/qemu-web.git * `<https://www.qemu.org/2017/02/04/the-new-qemu-website-is-up/>`_ A 'git-publish' utility was created to make above process less cumbersome, and is highly recommended for making regular contributions, or even just for sending consecutive patch series revisions. It also requires a working 'git send-email' setup, and by default doesn't automate everything, so you may want to go through the above steps manually for once. For installation instructions, please go to: * `<https://github.com/stefanha/git-publish>`_ The workflow with 'git-publish' is: .. code-block:: shell $ git checkout master -b my-feature $ # work on new commits, add your 'Signed-off-by' lines to each $ git publish Your patch series will be sent and tagged as my-feature-v1 if you need to refer back to it in the future. Sending v2: .. code-block:: shell $ git checkout my-feature # same topic branch $ # making changes to the commits (using 'git rebase', for example) $ git publish Your patch series will be sent with 'v2' tag in the subject and the git tip will be tagged as my-feature-v2. Bug reporting ============= The QEMU project uses GitLab issues to track bugs. Bugs found when running code built from QEMU git or upstream released sources should be reported via: * `<https://gitlab.com/qemu-project/qemu/-/issues>`_ If using QEMU via an operating system vendor pre-built binary package, it is preferable to report bugs to the vendor's own bug tracker first. If the bug is also known to affect latest upstream code, it can also be reported via GitLab. For additional information on bug reporting consult: * `<https://wiki.qemu.org/Contribute/ReportABug>`_ ChangeLog ========= For version history and release notes, please visit `<https://wiki.qemu.org/ChangeLog/>`_ or look at the git history for more detailed information. Contact ======= The QEMU community can be contacted in a number of ways, with the two main methods being email and IRC: * `<mailto:qemu-devel@nongnu.org>`_ * `<https://lists.nongnu.org/mailman/listinfo/qemu-devel>`_ * #qemu on irc.oftc.net Information on additional methods of contacting the community can be found online via the QEMU website: * `<https://wiki.qemu.org/Contribute/StartHere>`_