linux-kernel - Re: [RFC][PATCH RT] rwsem_rt: Another (more sane) approach to mulit reader rt locks

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Date:	Thu, 17 May 2012 18:17:47 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	RT <linux-rt-users@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <williams@...hat.com>
Subject: Re: [RFC][PATCH RT] rwsem_rt: Another (more sane) approach to mulit
 reader rt locks

On Thu, 2012-05-17 at 08:47 -0700, Paul E. McKenney wrote:
> On Thu, May 17, 2012 at 05:32:59PM +0200, Peter Zijlstra wrote:
> > On Thu, 2012-05-17 at 08:18 -0700, Paul E. McKenney wrote:
> > > Some researchers at MIT RCU-ified this lock:
> > > 
> > > http://people.csail.mit.edu/nickolai/papers/clements-bonsai.pdf 
> > 
> > Ah, as have I [1].. and they seem to have gotten about as far as I have,
> > which means almost there but not quite [2] :-)
> 
> I had forgotten about that -- that was the first call for call_srcu(),
> if I remember correctly.
> 
> > The most interesting case is file maps and they simply ignored those.
> > While I appreciate that from an academic pov, -- they can still write a
> > paper on the other interesting bits -- I don't really like it from a
> > practical point.
> > 
> > [1] https://lkml.org/lkml/2010/1/4/257
> > [2] https://lkml.org/lkml/2010/1/4/532
> 
> Hmmm...  Do the recent dcache changes cover some of the things that
> Linus called out?  Probably not, but some at least.

No, and the points viro made:

  https://lkml.org/lkml/2010/1/5/194

are still very much an issue, you really don't want to do fput() from an
asynchronous context. Which means you have to do synchronize_rcu() or
similar from munmap() which will be rather unpopular :/

Since we should not use per-cpu data for either files or processes
(there are simply too many of those around) the alternative is
horrendously hideous things like:

  https://lkml.org/lkml/2010/1/6/136

which one cannot get away with either.

The whole thing is very vexing indeed since all of this is only needed
for ill-behaved applications since a well-constructed application will
never fault in a range it is concurrently unmapping.

Most annoying.
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