mutex vs. local irqs (Was: 2.6.18 -mm merge plans)

[Benjamin Herrenschmidt]

> work-around-ppc64-bootup-bug-by-making-mutex-debugging-save-restore-irqs.patch
> kernel-kernel-cpuc-to-mutexes.patch
> 
>  ug.  We cannot convert the cpu.c semaphore into a mutex until we work out
>  why power4 goes titsup if you enable local interrupts during boot.

What is the exact problem ? Some mutex is forcing local irqs enabled
before init_IRQ() ? (Before the normal enabling of IRQ done by
init/main.c just after init_IRQ() more precisely ?)

This is bad for any architecture. Basically, at this point, the
interrupt controller can be in _any_ state, with possible pending
interrupts for whatever sources, etc...

As we discussed before, that problem should really be fixed in the mutex
code by not hard-enabling.

There is an incredible amount of crap that could be cleaned up for
example by re-ordering a bit the init code and making things like slab
available before init_IRQ/time_init etc... but all of those will break
because of that.

In addition, even without that re-ordering, I'm pretty sure we are
hitting semaphores/mutexes early, before init_IRQ(), already and if not
in generic code, in arch code somewhere down the call stacks.

I don't think that whole pile of problems lurking around the corner is
worth the couple of cycles saved by hard-enabling irq in the mutex
instead of doing a save/restore.

Ben.


-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/