Re: recursive spinlocks. Shoot.

[Nick Piggin]

On Thu, May 22, 2003 at 06:42:15AM +0200, Peter T. Breuer wrote:
> "Nick Piggin wrote:"
> > Locking is an implementation issue, and as such I think you'll have
> > to come up with a real problem or some real code. You must have some
> > target problem in mind?
> 
> I'll butt back in here.
> 
snip

> 
> The result was a call from the block driver to the md driver with a
> lock held, and a rather unexpected call back from the md driver that
> impotently tried to take the same lock.
> 
> Same thread.
>

OK, in this case, it didn't sound like your block driver
expected to be re-entered. Lucky the problem immediately
caused a deadlock ;)

More seriously, lets say you get around the above with
recursive locks:

Thread 1			Thread 2 (on another cpu)
enter the block driver
take the bd lock
				issue an md ioctl
				take the md lock
				enter the block driver
				spin on bd lock
automatically call md ioctl
spin on md lock

So the problem needs a rethink.