Re: [patch 0/5] [RFC] Conversion of reverse map locks to semaphores

[Christoph Lameter <clameter@sgi.com>]

On Fri, 27 Jun 2008, Peter Zijlstra wrote:

> > Also it seems that a semaphore helps RT and should avoid busy spinning
> > on systems where these locks experience significant contention.
> 
> Please be careful with the wording here. Semaphores are evil esp for RT.
> But luckily you're referring to a sleeping RW lock, which we call
> RW-semaphore (but is not an actual semaphore).
> 
> You really scared some people saying this ;-)

Well we use the term semaphore for sleeping locks in the kernel it seems.

Maybe you could get a patch done that renames the struct to 
sleeping_rw_lock or so? That would finally clear the air. This is the 
second or third time we talk about a semaphore not truly being a 
semaphore.

> Depending on the contention stats you could try an adaptive spin on the
> readers. I doubt adaptive spins on the writer would work out well, with
> the natural plenty-ness of readers..

That depends on the frequency of lock taking and the contention. If you 
have a rw lock then you would assume that writers are rare so this is 
likely okay.


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