Re: Role of process priority in locking mechanism

From: Casper H.S. Dik (Casper.Dik_at_Sun.COM)
Date: 12/23/04 

Date: 23 Dec 2004 18:34:41 GMT

kamalp@acm.org (Kamal R. Prasad) writes:

>splxxx() are a set of routines -which provide a priority level(xxx),
>which preempts other lower priority interrupts from preempting the
>code. They are used throughout the UNIX kernel, but they do not
>necessarily have anything to do with the hw. You could use splxxx()
>within an implementation of a syscall() , a device's read(), mmap(),
>ioctl() etc.. Its just to ensure atomicity of execution of a certain
>fragment of a code block.

Except that it doesn't in a Solaris kernel (there is more than one CPU)
and you also block progress of all unrelated threads; in that it
is a very primitive, coarse grained, lock.
(splnet(), e.g., locks at the network interrupts so you can safely
do administrative in the network stack)

>a primitive lock would be something like a spinlock().

Why is that primitive? Spinlocks are used a lot in MP systems
for short term waits (the Solaris kernel spins when it tries to
grab a lock held by a thread currently running on a CPU); it assumes
the lock will be short term. (It is an adaptive lock)

>Well -whether the kernel is multi-threaded or not does not ahve much
>to do with the nature of locks. Perhaps you mean, MP has a lot to with
>the implementation of locks.

>then how will you implement splnet()? Its not related to any driver.

You don't; the Solaris kernel does not have any interrupt level
manipulation routines available for use by driver writers.

>netbsd, freebsd etc.. are all examples of multi-threaded kernels that
>dont work that way You are describing an implementation.

Yes, I did not claim otherwise; threaded implementations which still
require spl*() calls are not MT-hot as those which do not.

Casper 

-- 
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to opinions held by my employer, Sun Microsystems.
Statements on Sun products included here are not gospel and may
be fiction rather than truth.
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