- 16 10月, 2012 2 次提交
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由 Daniel P. Berrange 提交于
Add two new APIs virLockSpaceNewPostExecRestart and virLockSpacePreExecRestart which allow a virLockSpacePtr object to be created from a JSON object and saved to a JSON object, for the purposes of re-exec'ing a process. As well as saving the state in JSON format, the second method will disable the O_CLOEXEC flag so that the open file descriptors are preserved across the process re-exec() Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>
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由 Daniel P. Berrange 提交于
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: NDaniel P. Berrange <berrange@redhat.com>
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