Re: floppy driver assumes INITIAL_JIFFIES == 0

[Stephen Hemminger]

On Thu, 3 Jun 2010 15:33:23 -0700 (PDT)
Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> 
> 
> On Thu, 3 Jun 2010, Linus Torvalds wrote:
> >
> > > So still a race that shows up with KVM (fast floppy?) and manifests
> > > as floppy_ready or reset_interrupt OOPS.
> > 
> > Yes, it's quite possible that the Linux floppy driver is simply broken by 
> > any floppy device that basically responds immediately to a command with an 
> > interrupt. And considering how few people use floppies, I do expect that 
> > driver to get _worse_ rather than better in the future.
> 
> Having looked at that driver some more, I can inf act pretty much 
> guarantee it. The locking is rather baroque. It has a "floppy_lock", but 
> that only protects certain small parts. In particular, it looks like the 
> irq handler and the timers do _not_ take it, and that's where most of the 
> real work is done.
> 
> And in fact, that does look broken. The interrupt handler really does a 
> "schedule_work()" to schedule the actual handler outside of irq context, 
> and I don't see any serialization between the timers that file and the 
> handler running.
> 
> That driver used to be this state machine that ran entirely from interrupt 
> context, where one interrupt handler would set the state for the next one 
> (that's what the "do_floppy" thing is for). But then it became bottom 
> halves, and now it's using schedule_work() instead - and at the same time, 
> the _timers_ haven't really changed. Those run in timer context, and can 
> thus interrupt the work thing.
> 
> It always was a disgusting driver. Now it's just even more so. And yes, 
> I'm sure it's full of races that are largely hidden by the fact that real 
> floppy hardware is so slow that you can never hit them.
> 
> Looking too much at that driver will cause PTSD.  I have to look away.

Thank you for confirming my suspicions.  Given the state of destruction
there, bug fixing is like playing Jenga.
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