Re: What is a sleeping spin locks?

[Rene Herman <rene.herman@xxxxxxxxxxxx>]

On 04-08-08 17:36, Peter Teoh wrote:

> I read about this sleeping spin lock:
>
> http://lwn.net/Articles/271817/
>
> What is that?

A marketing oxymoron in the same style as, say, "voluntary preemption".

Ofcourse, "sleeping spinlocks" do not exist. Although adaptive spinlocks 
which spin for a while before giving up and going to sleep might sort of 
deserve the name, it's still no longer a spinlock if it goes to sleep 
(and adaptive spinlocks might be the current -rt thing or not, I don't 
know).

> I don't quite understand.   Normal spin lock is poll-based, but
> sleeping spin lock is not, then how does it differed from mutex then?

In principle, they do not. I've never looked at the RT stuff in detail 
but I believe that in practice, it's specifically an _RT_ mutex, which 
features priority inversion avoidance over regular mutexes.

Note that the -rt kernel for example also makes interrupt handlers 
scheduled entities (ie, they run alongside anything else and are just 
part of the normal locking dance) so the locking rules and what you can 
and cannot do change significantly under -rt. Understanding how and when 
these "sleeping spinlocks" are safe requires a fuller digging into the 
specifics of -rt therefore.

If you do, be sure to post back the full story... ;-/

Rene.

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