On Wed, Jul 22, 2009 at 9:04 PM, Venkatesh Srinivas<m...@acm.jhu.edu> wrote: > I noticed that p9p has threadpin() and threadunpin() in its thread > library... they claim to make the current thread the only one runnable > in this proc. I'm failing to see the purpose of these... a thread is > not subject to preemptive scheduling, it can achieve the same effect > by not calling yield(), right? > Also, these two functions aren't in p9's libthread... is there any > reason why not (assuming those two have a purpose)?
Yes and yes. Unfortunately, it is sometimes impossible to avoid calling yield. On Plan 9, the graphics library uses writes to /dev/draw and the kernel manages simultaneous access by multiple threads. In Plan 9 port, everything happens in user space and there are qlocks to help make things thread safe. Unfortunately the fact that the qlocks can reschedule inside a proc makes them different from a write system call. Rather than rewrite all the draw code to remove the assumption that /dev/draw writes do not reschedule, I made the simulation of /dev/draw also not reschedule. It is not a good approach in general, which is why the functions are undocumented. http://code.swtch.com/plan9port/src/tip/src/libdraw/drawclient.c#cl-189 Russ
