linux-kernel - Re: [PATCH] SLUB use cmpxchg

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Message-Id: <1188330413.11709.27.camel@lappy>
Date:	Tue, 28 Aug 2007 21:46:52 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Christoph Lameter <clameter@....com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	mingo@...hat.com
Subject: Re: [PATCH] SLUB use cmpxchg_local


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. ]

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