linux-kernel - Re: [PATCH] fix a race condition in cancelable mcs spinlocks

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Date:	Mon, 02 Jun 2014 08:27:38 -0700
From:	Jason Low <jason.low2@...com>
To:	Waiman Long <waiman.long@...com>
Cc:	Mikulas Patocka <mpatocka@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>, jejb@...isc-linux.org,
	deller@....de, John David Anglin <dave.anglin@...l.net>,
	linux-parisc@...r.kernel.org, linux-kernel@...r.kernel.org,
	chegu_vinod@...com, paulmck@...ux.vnet.ibm.com, tglx@...utronix.de,
	riel@...hat.com, akpm@...ux-foundation.org, davidlohr@...com,
	hpa@...or.com, andi@...stfloor.org, aswin@...com,
	scott.norton@...com
Subject: Re: [PATCH] fix a race condition in cancelable mcs spinlocks

On Mon, 2014-06-02 at 10:14 -0400, Waiman Long wrote:
> On 06/01/2014 01:53 PM, Mikulas Patocka wrote:
> >   struct optimistic_spin_queue {
> > -	struct optimistic_spin_queue *next, *prev;
> > +	atomic_pointer_t next;
> > +	struct optimistic_spin_queue *prev;
> >   	int locked; /* 1 if lock acquired */
> >   };
> 
> Is there a way to do it without changing the pointer type? It will make 
> the code harder to read and understand.

I agree that it would be nice if there is a way to fix this without
changing the pointer type of "next". The change of the type to
atomic_pointer_t might make it less obvious what "next" is for. This is
then compounded with "prev" being kept as a pointer to
optimistic_spin_queue, which can further make it appear as if "next" may
potentially point to something different.


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