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Daniel P. Berrangé
e1c32120ce
The virshtest program testPipeFeeder method is doing this:
mkfifo("test.fifo", 0600) ;
int fd = open("test.fifo", O_RDWR);
char buf[...];
memset(buf, 'a', sizeof(buf));
write(fd, buf, sizeof(buf)) == sizeof(buf));
close(fd);
while the the 'virsh' child process then ends up doing:
fd = open("test.fifo", O_RDONLY);
read(fd, buf, sizeof(buf)) == sizeof(buf));
close(fd);
The 'virsh' code hangs on open() on at least ppc64 and some other
arches. It can be provoked to hang even on x86 by reducing the size of
the buffer. It can be prevented from hanging on ppc64 by increasing the
size of the buffer.
What is happening is a result of differing page sizes, altering the
overall pipe capacity size, since pipes on linux default to 16 pages
in size and thus have architecture specific capacity when measured
in bytes.
* On x86, testPipeFeeder opens R+W, tries to write 140kb and
write() blocks because the pipe is full. This gives time for
virsh to start up, and it can open the pipe for O_RDONLY
since testPipeFeeder still has it open for write. Everything
works as intended.
* On ppc64, testPipeFeeder opens R+W, tries to write 140kb
and write() succeeds because the larger 64kb page size
resulted in greater buffer capacity for the pipe. It thus
quickly closes the pipe, removing the writer, and triggering
discard of all the unread data. Now virsh starts up, tries
to open the pipe for O_RDONLY and blocks waiting for a new
writer to open it, which will never happen. Meson kills
the test after 30 seconds.
NB, every now & then, it will not block because virsh starts
up quickly enough that testPipeFeeder has not yet closed the
write end of the pipe, giving the illusion of correctness.
The key flaw here is that it should not have been using O_RDWR
in testPipeFeeder. Synchronization is required such that both
virsh and testPipeFeeder have their respective ends of the pipe
open before any data is sent. This is trivially arranged by
using O_WRONLY in testPipeFeeder.
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
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==============================
Libvirt API for virtualization
==============================
Libvirt provides a portable, long term stable C API for managing the
virtualization technologies provided by many operating systems. It
includes support for QEMU, KVM, Xen, LXC, bhyve, Virtuozzo, VMware
vCenter and ESX, VMware Desktop, Hyper-V, VirtualBox and the POWER
Hypervisor.
For some of these hypervisors, it provides a stateful management
daemon which runs on the virtualization host allowing access to the
API both by non-privileged local users and remote users.
Layered packages provide bindings of the libvirt C API into other
languages including Python, Perl, PHP, Go, Java, OCaml, as well as
mappings into object systems such as GObject, CIM and SNMP.
Further information about the libvirt project can be found on the
website:
https://libvirt.org
License
=======
The libvirt C API is distributed under the terms of GNU Lesser General
Public License, version 2.1 (or later). Some parts of the code that are
not part of the C library may have the more restrictive GNU General
Public License, version 2.0 (or later). See the files ``COPYING.LESSER``
and ``COPYING`` for full license terms & conditions.
Installation
============
Instructions on building and installing libvirt can be found on the website:
https://libvirt.org/compiling.html
Contributing
============
The libvirt project welcomes contributions in many ways. For most components
the best way to contribute is to send patches to the primary development
mailing list. Further guidance on this can be found on the website:
https://libvirt.org/contribute.html
Contact
=======
The libvirt project has two primary mailing lists:
* users@lists.libvirt.org (**for user discussions**)
* devel@lists.libvirt.org (**for development only**)
Further details on contacting the project are available on the website:
https://libvirt.org/contact.html