Re: bkl cleanup in do_sysctl

[Hans Reiser]

Lee Revell wrote:

> On Tue, 2004-08-10 at 13:28, Dave Hansen wrote:
>
>  
>
> > Remember that the BKL isn't a plain-old spinlock.  You're allowed to
> > sleep while holding it and it can be recursively held, which isn't true
> > for other spinlocks.
> >
> > So, if you want to replace it with a spinlock, you'll need to do audits
> > looking for sysctl users that might_sleep() or get called recursively
> > somehow.  The might_sleep() debugging checks should help immensely for
> > the first part, but all you'll get are deadlocks at runtime for any
> > recursive holders.  But, those cases are increasingly rare, so you might
> > luck out and not have any.  
> >
> > Or, you could just make it a semaphore and forget about the no sleeping
> > requirement.  
> >
> >    
>
> Someone once suggested that newbies who show up on LKML wanting to learn
> kernel hacking should be assigned to find one use of the BKL and replace
> it with proper locking.  Something similar worked very well with my
> previous employer, before giving someone root, new hires would first be
> assigned some task like writing a script to take the user account
> database and generate a report of old accounts on a bunch of machines,
> or rewrite the RADIUS accounting scripts, where the point was really to
> get them familiar with the system.
>
> This way, even if they come back with a totally botched fix, someone
> will probably just post a correct one.  We could get rid of the BKL very
> soon, I count only 247 files with lock_kernel in them.
>
> For example reiserfs uses the BKL for all write locking (!), but it
> probably would not be too hard to fix, because you can just look at
> another filesystem that has proper locking.
>  
Wrong. ;-)  Balancing makes it way hard.  Use reiser4.  That has very 
sophisticated locking that pushes the research envelope, if you want to 
read code to learn about locking.....

> Maybe this should be added to the FAQ:
>
> Q:  I want to hack the kernel, and I *think* I know what I am doing. 
> Where do I start?
>
> Lee
>
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