Re: [PATCH] SLUB use cmpxchg_local

Peter Zijlstra
2007-08-28T19:47:38Z

On Tue, 2007-08-28 at 12:36 -0700, Christoph Lameter wrote:
> On Tue, 28 Aug 2007, Peter Zijlstra wrote:
>
> > On Mon, 2007-08-27 at 15:15 -0700, Christoph Lameter wrote:
> > > Hmmmm. One wild idea would be to use a priority futex for the slab lock?
> > > That would make the slow paths interrupt safe without requiring interrupt
> > > disable? Does a futex fit into the page struct?
> >
> > Very much puzzled at what you propose. in-kernel we use rt_mutex (has
> > PI) or mutex, futexes are user-space. (on -rt spinlock_t == mutex ==
> > rt_mutex)
> >
> > Neither disable interrupts since they are sleeping locks.
> >
> > That said, on -rt we do not need to disable interrupts in the allocators
> > because its a bug to call an allocator from raw irq context.
>
> Right so if a prioriuty futex

futex stands for Fast Userspace muTEX, please lets call it a rt_mutex.

> would have been taken from a process
> context and then an interrupt thread (or so no idea about RT) is scheduled
> then the interrupt thread could switch to the process context and complete
> the work there before doing the "interrupt" work. So disabling interrupts
> is no longer necessary.

-rt does all of the irq handler in thread (process) context, the hard
irq handler just does something akin to a wakeup.

These irq threads typically run fifo/50 or simething like that.

[ note that this allows a form of irq priorisation even if the hardware
doesn't. ]