Re: console spin_lock

[Andrew Morton]

James Simmons wrote:
> ...
> By you saying couldn't be acquired from interrupt context do you mean
> from a process context or do you mean it failed to aquire it while in
> the interrupt context?

Actually, printk() must always use __down_trylock().

> > - Get rid of console_tasklet.  Do it in process context callback
> >   or just do it synchronously.
> 
> What about multidesktop systems? I have vgacon and mdacon working fine
> along each other. Each one has their own tasklet to allow them to work
> independent of each other. Meaning no race condition when both VC switch
> at the same time.

Ah.  Thanks. That stuff was actually design-from-memory :)  I'll take
a closer look when I have something other than a clockwork computer.
 
> > Assumption:
> > - Once the system is up and running, it's always safe to
> >   call down() when in_interrupt() returns false - probably
> >   not the case in parts of the exit path - tough.
> 
> Don't forget the idle_task case as well. exit path?

This statement of mine was grade-A bollocks.  printk cannot of
course call down().  It needs to use __down_trylock and buffer
it up if it fails. (faster, too!)

The subtler problem will be interrupt-capable drivers which
do a bare spin_lock() to serialise wrt their interrupt routines,
relying upon interrupts being disabled.  They'll be deadlocky
and will need changing.  That's trivial to find and fix though.

Anyway, this was just a heads-up that I'll be looking at
this stuff.  Please allow me a week or so to provide
some substance.  I read that the fbdev developers have
been seeking a fix for this for some time, so it seems
worth some effort.