Re: [9fans] p9p threadpin() / threadunpin() ?

[Russ Cox <rsc@swtch.com>]

On Wed, Jul 22, 2009 at 9:04 PM, Venkatesh Srinivas<me@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