linux-kernel - Re: Using sched_clock for mmio-trace

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Date:	Sun, 18 Feb 2007 18:20:43 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Andi Kleen <andi@...stfloor.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Jeff Muizelaar <jeff@...idigm.net>,
	linux-kernel@...r.kernel.org
Subject: Re: Using sched_clock for mmio-trace

On Sat, Feb 17, 2007 at 04:19:58PM +0100, Thomas Gleixner wrote:
> On Sat, 2007-02-17 at 15:56 +0100, Andi Kleen wrote:
> > > This is one of the reasons why we don't just use good old
> > > do_gettimeofday(), since it takes locks and can lead to lock recursion
> > > if parts of itself are probed.
> > 
> > do_gettimeofday doesn't take locks.
> > 
> > Only restriction is that you can't single step it with long 
> > pauses between instructions.
> 
> Err, it uses read side of xtime lock, so you can not call it from a
> place which write locks xtime lock.

Err, you can -- seqlocks never deadlock.

The only thing that doesn't work is to single step with long enough
pauses with interrupts on inbetween that the sequence numbers increase:
you get a livelock then.

-Andi
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