investigation of Giant usage in kernel

[Robert Watson]

On Sun, 4 Mar 2007, Divacky Roman wrote:

> I looked at where Giant is held in the kernel and I found these interesting 
> things:
>
> 1) in fs/fifofs/fifo_vnops.c we lock Giant when calling sorecieve()/sosend() 
> this is a bandaid for fixing a race that doesnt have to exist anymore. ie. 
> it needs some testing and can be remvoed

Hmm.  I think that conclusion is a bit premature.  Per our conversation on 
IRC, the workaround was added back prior to a release due to our being unable 
to resolve a very difficult to debug race condition.  There is no evidence the 
race doesn't exist anymore: what is needed is testing to determine if it does 
or not.  The race condition occurred under high make -j load on SMP; FYI, make 
uses a fifo to implement a concurrency limiting token scheme in order to bound 
total simultaneous jobs despite many make instances running.

I've CC'd Kris since I know that he was able to reproduce the problem, so 
might be able to provide advice on how to do so.

Robert N M Watson
Computer Laboratory
University of Cambridge