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


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/

Relevant Pages

  • Re: [PATCH 1/19] MUTEX: Introduce simple mutex implementation
    ... > a mutex was a sensible implementation tradeoff. ... are really counting semaphores, ... >> acceptable patch that introduces a separate data structure. ... > general semaphore, because a mutex has stronger invariants. ...
    (Linux-Kernel)
  • Re: [PATCH 1/19] MUTEX: Introduce simple mutex implementation
    ... >> A counting semaphore is NOT a perfectly fine mutex, ... >> People are indeed unhappy with the naming, ...
    (Linux-Kernel)
  • [patch 00/15] Generic Mutex Subsystem
    ... generic mutex subsystem that we have in the -rt kernel, ... 'simple mutex' code recently posted by David Howells.) ... 'struct mutex' is 16 bytes. ... than the semaphore based kernel, _and_ it also had 2.8 times less CPU ...
    (Linux-Kernel)
  • Re: [PATCH 1/19] MUTEX: Introduce simple mutex implementation
    ... I thought you were considering naming unhappiness to be a reason ... _for_ the mutex change. ... a real double aquire, and not a race due to lock ordering) easy to see, ... a hung semaphore isn't exactly hard to debug). ...
    (Linux-Kernel)
  • Re: [PATCH 1/19] MUTEX: Introduce simple mutex implementation
    ... > Outside of the arch directories, yes; but I don't know that I've made the ... I'm in the crowd that thinks that the mutex downs and ups should be ... that should be a semaphore. ... > I've attempted to review everything in 2.6.15-rc5 outside of most of the archs. ...
    (Linux-Kernel)