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Alex Bennée reported a ThreadSanitizer warning about a plain concurrent
access to ram_list [1]. Ensure the concurrent accesses to ram_list are
properly synchronized with atomic accesses, mutexes, or RCU.
First, the plain assignments of ram_list.mru_block are replaced with
qatomic_set(). A comment in qemu_get_ram_block() explains why the
ordering requirement is relaxed, but it still needs to be atomically
accessed. include/qemu/atomic.h says:
> The C11 memory model says that variables that are accessed from
> different threads should at least be done with __ATOMIC_RELAXED
> primitives or the result is undefined. Generally this has little to
> no effect on the generated code but not using the atomic primitives
> will get flagged by sanitizers as a violation.
Second, ram_list.version accesses are replaced with atomic operations
or protected with a mutex. Unlike ram_list.mru_block, ram_list.version
has tighter ordering requirements for one of its goals: ensuring that
the reader-held rs->last_seen_block value is invalidated whenever a RAM
block is reclaimed between two RCU reader critical sections. Below are
steps a reader and an updater follow:
Reader:
R-1. Enter the first RCU read-side critical section:
R-1-1. rs->last_version = qatomic_load_acquire(&ram_list.version)
R-1-2. rs->last_seen_block = an element of ram_list.blocks
R-2. Enter the second RCU read-side critical section:
R-2-1. if (qatomic_read(&ram_list.version) != rs->last_version)
R-2-2. rs->last_seen_block = NULL
Updater:
W-1. Enter a ram_list.mutex critical section
W-1-1. Update ram_list.blocks
W-1-2. qatomic_store_release(&ram_list.version, ram_list.version + 1)
W-2. Enter another ram_list.mutex critical section
W-2-1. QLIST_REMOVE_RCU(block, next)
W-2-2. qatomic_store_release(&ram_list.version, ram_list.version + 1)
W-2-3. call_rcu(block, reclaim_ramblock, rcu)
W-1-2 represents the write observed by R-1-1.
ram_list.version is read non-atomically on the update side because the
update side is serialized with ram_list.mutex. The other ram_list
accesses in these steps are reasoned about in two cases.
When the grace period of W-2-3 contains R-2:
qatomic_load_acquire() at R-1-1 and qatomic_store_release() at W-1-2
enforce the following ordering:
W-1-1 -> W-1-2 -> R-1-1 -> R-1-2
The value of ram_list.blocks stored by W-1-1 or a newer value that
was loaded by R-1-2 is still valid because of the grace period.
When the grace period of W-2-3 ends before R-2:
call_rcu() at W-2-3 and the read-side critical section at R-2 ensure
the following ordering:
W-2-2 -> W-2-3 -> the grace period -> R-2 -> R-2-1
The value of ram_list.version stored by W-2-2 or a newer value that
was loaded by R-2-1 differs from rs->last_version and the reader
invalidates rs->last_seen_block.
Together, these steps ensure that rs->last_seen_block is invalidated
whenever necessary. With added atomic operations, pre-existing memory
barriers are no longer necessary and are removed.
Any other ram_list accesses are already properly synchronized.
[1] https://lore.kernel.org/qemu-devel/878q9fbmap.fsf@draig.linaro.org/
Signed-off-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/20260523-tsan-v1-1-07d5eb9dcaa2@rsg.ci.i.u-tokyo.ac.jp
Signed-off-by: Peter Xu <peterx@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>`_