linux-kernel - Re: [RFC][PATCH 0/5] arch: atomic rework

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Message-ID: <20140220092049.GA22688@linux.vnet.ibm.com>
Date:	Thu, 20 Feb 2014 01:20:49 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Torvald Riegel <triegel@...hat.com>,
	Will Deacon <will.deacon@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
	David Howells <dhowells@...hat.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"gcc@....gnu.org" <gcc@....gnu.org>
Subject: Re: [RFC][PATCH 0/5] arch: atomic rework

On Thu, Feb 20, 2014 at 12:30:32AM -0800, Paul E. McKenney wrote:
> On Wed, Feb 19, 2014 at 08:43:14PM -0800, Linus Torvalds wrote:

[ . . . ]

> So, if you make one of two changes to your example, then I will agree
> with you.  The first change is to have a real data dependency between
> the read of "a" and the second read:
> 
>  - Initial state:
> 
>    a = &c, b = 0; c = 0;
> 
>  - Thread 1 ("consumer"):
> 
>     if (atomic_read(&a, consume))

And the above should be "if (atomic_read(&a, consume) != &c)".  Sigh!!!

							Thanx, Paul

>          return *a;
>     /* not yet initialized */
>     return -1;
> 
>  - Thread 2 ("initializer"):
> 
>      b = some_value_lets_say_42;
>      /* We are now ready to party */
>      atomic_write(&a, &b, release);

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