linux-kernel - Re: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup

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Date:	Sun, 18 Apr 2010 13:55:17 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Avi Kivity <avi@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	akpm@...ux-foundation.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	David Miller <davem@...emloft.net>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup_fast()
 implementation

On Sun, 2010-04-18 at 06:55 -0700, Paul E. McKenney wrote:
> On Sat, Apr 17, 2010 at 10:06:36PM -0500, James Bottomley wrote:
> > On Fri, 2010-04-16 at 09:45 -0700, Paul E. McKenney wrote:
> > > o	mutex_lock(): Critical sections need not guarantee
> > > 	forward progress, as general blocking is permitted.
> > 
> > This isn't quite right.  mutex critical sections must guarantee eventual
> > forward progress against the class of other potential acquirers of the
> > mutex otherwise the system will become either deadlocked or livelocked.
> 
> If I understand you correctly, you are saying that it is OK for a given
> critical section for a given mutex to fail to make forward progress if
> nothing else happens to acquire that mutex during that time.  I would
> agree, at least I would if you were to further add that the soft-lockup
> checks permit an additional 120 seconds of failure to make forward progress
> even if something -is- attempting to acquire that mutex.

Yes ... I was thinking of two specific cases: one is wrong programming
of lock acquisition where the system deadlocks; the other is doing silly
things like taking a mutex around an event loop instead of inside it so
incoming events prevent forward progress and the system livelocks, but
there are many other ways of producing deadlocks and livelocks.  I just
couldn't think of a concise way of saying all of that but I didn't want
a statement about mutexes to give the impression that anything goes.

I've got to say that I also dislike seeing any form of sleep within a
critical section because it's just asking for a nasty entangled deadlock
which can be very hard to sort out.  So I also didn't like the statement
"general blocking is permitted"

> By my standards, 120 seconds is a reasonable approximation to infinity,
> hence my statement above.
> 
> So, would you agree with the following as a more precise statement?
> 
> o	mutex_lock(): Critical sections need not guarantee
> 	forward progress unless some other task is waiting
> 	on the mutex in question, in which case critical sections
> 	should complete in 120 seconds.

Sounds fair.

James


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