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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.
eca72d4759
The previously introduced virFile{Lock,Unlock} APIs provide a way to acquire/release fcntl() locks on individual files. For unknown reason though, the POSIX spec says that fcntl() locks are released when *any* file handle referring to the same path is closed. In the following sequence threadA: fd1 = open("foo") threadB: fd2 = open("foo") threadA: virFileLock(fd1) threadB: virFileLock(fd2) threadB: close(fd2) you'd expect threadA to come out holding a lock on 'foo', and indeed it does hold a lock for a very short time. Unfortunately when threadB does close(fd2) this releases the lock associated with fd1. For the current libvirt use case for virFileLock - pidfiles - this doesn't matter since the lock is acquired at startup while single threaded an never released until exit. To provide a more generally useful API though, it is necessary to introduce a slightly higher level abstraction, which is to be referred to as a "lockspace". This is to be provided by a virLockSpacePtr object in src/util/virlockspace.{c,h}. The core idea is that the lockspace keeps track of what files are already open+locked. This means that when a 2nd thread comes along and tries to acquire a lock, it doesn't end up opening and closing a new FD. The lockspace just checks the current list of held locks and immediately returns VIR_ERR_RESOURCE_BUSY. NB, the API as it stands is designed on the basis that the files being locked are not being otherwise opened and used by the application code. One approach to using this API is to acquire locks based on a hash of the filepath. eg to lock /var/lib/libvirt/images/foo.img the application might do virLockSpacePtr lockspace = virLockSpaceNew("/var/lib/libvirt/imagelocks"); lockname = md5sum("/var/lib/libvirt/images/foo.img"); virLockSpaceAcquireLock(lockspace, lockname); NB, in this example, the caller should ensure that the path is canonicalized before calculating the checksum. It is also possible to do locks directly on resources by using a NULL lockspace directory and then using the file path as the lock name eg virLockSpacePtr lockspace = virLockSpaceNew(NULL); virLockSpaceAcquireLock(lockspace, "/var/lib/libvirt/images/foo.img"); This is only safe to do though if no other part of the process will be opening the files. This will be the case when this code is used inside the soon-to-be-reposted virlockd daemon Signed-off-by: Daniel P. Berrange <berrange@redhat.com> |
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.gnulib@2a9edc6f2b | ||
build-aux | ||
daemon | ||
docs | ||
examples | ||
gnulib | ||
include | ||
m4 | ||
po | ||
python | ||
src | ||
tests | ||
tools | ||
.dir-locals.el | ||
.gitignore | ||
.gitmodules | ||
.mailmap | ||
AUTHORS | ||
autobuild.sh | ||
autogen.sh | ||
bootstrap | ||
bootstrap.conf | ||
cfg.mk | ||
ChangeLog-old | ||
configure.ac | ||
COPYING.LIB | ||
HACKING | ||
libvirt.pc.in | ||
libvirt.spec.in | ||
Makefile.am | ||
Makefile.nonreentrant | ||
mingw-libvirt.spec.in | ||
README | ||
README-hacking | ||
run.in | ||
TODO |
LibVirt : simple API for virtualization Libvirt is a C toolkit to interact with the virtualization capabilities of recent versions of Linux (and other OSes). It is free software available under the GNU Lesser General Public License. Virtualization of the Linux Operating System means the ability to run multiple instances of Operating Systems concurrently on a single hardware system where the basic resources are driven by a Linux instance. The library aim at providing long term stable C API initially for the Xen paravirtualization but should be able to integrate other virtualization mechanisms if needed. Daniel Veillard <veillard@redhat.com>