linux-kernel - Re: preempt-rt, need old style rwlocks for systemtap

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Date:	Sat, 03 May 2008 16:05:07 -0600
From:	"Peter W. Morreale" <pmorreale@...ell.com>
To:	"Frank Ch. Eigler" <fche@...hat.com>
Cc:	Clark Williams <williams@...hat.com>, linux-kernel@...r.kernel.org,
	linux-rt-users@...r.kernel.org
Subject: Re: preempt-rt, need old style rwlocks for systemtap

On Sat, 2008-05-03 at 13:16 -0400, Frank Ch. Eigler wrote:
> Hi -
> 
> On Sat, May 03, 2008 at 09:34:16AM -0500, Clark Williams wrote:
> > [...]
> > > It has come to my attention that the preempt-rt patch suite
> > > deliberately defeats the potential concurrency intended by systemtap's
> > > use of rwlocks to permit concurrent readers [...]
> 
> > The reason it "defeats" the concurrent behavior is that it's really
> > complicated to have concurrent readers with Priority Inheritance, so
> > the initial cut of rtmutexes serialized all lock accesses. [...]
> 
> Do you believe priority inheritance to be an essential property of
> every use of these primitives, regardless of the nature of the
> specifical critical sections being protected?  Is there no way & need
> to opt out of the extra machinery?
> 
> - FChE

Yes, for (at least) two reasons. And No.

Without PI the best case is indeterminate latencies.  The worst case is
deadlock.  Please read Steven's excellent description of the issues in
the Documentation directory of a kernel tree.   


Best,
-PWM


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