Re: BKL removal

[Dave Hansen <haveblue@us.ibm.com>]

Alexander Viro wrote:
> 
> On Tue, 9 Jul 2002, Arnaldo Carvalho de Melo wrote:
> 
>>Em Tue, Jul 09, 2002 at 02:47:49PM -0700, Robert Love escreveu:
>>
>>>On Tue, 2002-07-09 at 07:44, Dave Hansen wrote:
>>>
>>>>The Stanford Checker or something resembling it would be invaluable 
>>>>here.  It would be a hell of a lot better than my litle patch!
>>>
>>>The Stanford Checker would be infinitely invaluable here -- agreed.
>>>
>>>Anything that can graph call chains and do analysis... we can get it to
>>>tell us exactly who and what.
> 
> Not anything.  It can be used to find problems (and be very helpful at that)
> but it can't be used to verify anything.
> 
> The problem is that checker doesn't (and cannot) cover all code paths -
> by the time when it comes into the game the source had already passed
> through through cpp.  In other words, depending on the configuration
> you might get very different results.

I have the feeling that the filesystems' use of lots of function 
pointers will add a large amount of complexity to whatever programming 
any checker would require.  Bill Irwin and I were discussing it and we 
have ways of getting around most of them, but there are _lots_ of 
special cases.

   "Proving" correctness would obviously be ideal, but in an imperfect 
world, what are your feelings on a runtime system for detecting 
"magical/bad" BKL use?  I'm not proposing my kludgy "printk if you're 
bad" stuff, but something with much less impact.  I would like to do 
something somewhat like profile=2.  During each lock_kernel(), the 
program counter (any maybe more) could be stored in an internal kernel 
structure and retrieved later via a /proc file, just like readprofile. 
    This wouldn't have intrusive printk messages, and would be able to 
be activated by either a command-line parameter, or something else in 
/proc.  If we had this in our development kernel, interested 
developers could pay attention to the output, while the normal kernel 
developer could simply ignore it.

> Normally it's not that bad, but "can this function block?" is very nasty
> in that respect - changes of configuration can and do affect that in
> non-trivial ways.

I also wonder how it handles things like kmalloc(), which can block 
depending on arguments.

-- 
Dave Hansen
haveblue@us.ibm.com