linux-kernel - Re: [PATCH RFC] locking: Add volatile to arch_spinlock

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Message-ID: <20141204070248.GJ25340@linux.vnet.ibm.com>
Date:	Wed, 3 Dec 2014 23:02:48 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-arch@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
	dvyukov@...gle.com, peterz@...radead.org,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH RFC] locking: Add volatile to arch_spinlock_t structures

On Wed, Dec 03, 2014 at 10:40:45PM -0800, Linus Torvalds wrote:
> On Dec 3, 2014 10:31 PM, "Linus Torvalds" <torvalds@...ux-foundation.org>
> wrote:
> >
> > So no, no, no. C got this wrong. Volatile data structures are a
> fundamental mistake and a bug.
> 
> BTW, I'm not at all interested in language lawyering and people who say
> "but but we can do x". A compiler that modifies adjacent fields because the
> standard leaves is open is a crap compiler, and we won't use it, or disable
> the broken optimization. It is wrong from a concurrency standpoint anyway,
> and adding broken volatiles is just making things worse.

Understood, for example, adjacent fields protected by different locks
as one example, where adjacent-field overwriting completely breaks even
very conservatively designed code.  Should be entertaining!  ;-)

							Thanx, Paul

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