duplicate read/write locks in net/pfil.c and netinet/ip_fw2.c

[Stephan Uphoff]

On Thu, 2005-08-18 at 10:31, Luigi Rizzo wrote:
> On Thu, Aug 18, 2005 at 10:18:33AM -0400, Stephan Uphoff wrote:
> > On Thu, 2005-08-18 at 03:57, Luigi Rizzo wrote:
> ...
> > > In fact i don't understand why you consider spinning and sleeping
> > > on a mutex two different things.
> > 
> > The major difference between sleeping (cv_wait,msleep,..) and blocking
> > on a mutex is priority inheritance.
> > If you need to be able to use (non-spin) mutexes while holding a
> > [R|W]LOCK and use a [R|W]LOCK while holding a (non-spin) mutex then you
> > need to implement priority inheritance for [R|W]LOCKs.
> 
> is that required (in FreeBSD, i mean) for algorithmic
> correctness or just for performance ?

Hi Luigi,

It is theoretically required since otherwise low priority user threads
(programs) could block system (interrupt) threads indefinitely long.

Example:
	Extreme low priority (nice?) thread A holds read/write 
        lock RW as reader
	Thread B is holding mutex M tries to acquire read/write lock 
        RW as writer and sleeps.
	Thread C with better priority than A runs and enters a 
        busy loop (in user space). 
	Interrupt thread preempts C and tries to acquire Mutex M.
	Interrupt priority is propagated to B BUT NOT TO A.
	Interrupt thread blocks on Mutex M.
	Thread C resumes and will block thread I forever if it 
        can keep a better priority than thread A.

In practice you would probably just see bad latency every now and then
and may never encounter a hang.

Stephan