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

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Message-ID: <20100418135500.GD3096@linux.vnet.ibm.com>
Date:	Sun, 18 Apr 2010 06:55:00 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	James Bottomley <James.Bottomley@...senPartnership.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 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.

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.

							Thanx, Paul
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