Zack's Kernel News

Article from Issue 102/2009

The Linux kernel mailing list comprises the core of Linux development activities. Traffic volumes are immense, often reaching ten thousand messages in a given week, and keeping up to date with the entire scope of development is a virtually impossible task for one person. One of the few brave souls to take on this task is Zack Brown.

Our regular monthly column keeps you abreast of the latest discussions and decisions, selected and summarized by Zack. Zack has been publishing a weekly online digest, the Kernel Traffic newsletter, for over five years now. Even reading Kernel Traffic alone can be a time-consuming task.

Linux Magazine now provides you with the quintessence of Linux kernel activities, straight from the horse's mouth.

exFAT Support

Someone pointed out on the list that Microsoft's exFAT filesystem seemed to be their answer to large portable flash drives and asked what, if anything, was being done in the Linux world to support the exFAT filesystem. Hirofumi Ogawa replied that he'd already written a read-only driver, but because of time constraints, he had not been working actively on adding write support. He posted his code, and H. Peter Anvin asked whether there were any filesystem specs available that didn't require signing away the ability to write code to them. Hirofumi replied that his own work had been based on reverse-engineering the filesystem on disk. Meanwhile, Alex Buell converted Hirofumi's patches to a standard kernel module that can be compiled outside of the kernel, just as long as the system has a recent kernel installed. The code is available for download at: http://www.munted.org.uk/programming/exfat.tar.bz2.

GPL, LGPL, and IBM IP

Mathieu Desnoyers wanted to release the userspace RCU code he worked on under the LGPL instead of the GPL so that proprietary code could link with it and use it. RCU, read-copy-update, is a library that ensures that data objects defined in the kernel do not appear undefined to other running code that tries to access them before the definition process has completed. Defining and initializing a struct, for example, could expose it in an incomplete form if the compiler or CPU tries to optimize the data assignments and puts the struct assignment itself ahead of the code assigning values to the variables within that struct. Mathieu's library makes protection against this available to user space. He wanted to know whether switching to the LGPL would be legal and acceptable to the Linux community.

Alan Cox pointed out that IBM owned the patent on the RCU idea and they had only released the patent for use in GPLed code, so Mathieu would need to get permission from them before proceeding.

Ktrace Sleeper Kernel Tracing

Jiaying Zhang created ktrace, a mechanism for tracing kernel events by inserting tracepoints in the kernel code. The reason for this innovation is that the existing markers code are deemed too heavy-weight. By simplifying the design, Jiaying and the other ktrace developers found significant performance enhancements. Some prototype code is posted.

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