linux-kernel - Re: [PATCH v2 1/1] mm: fix the theoretical compound_lock() vs prep_new

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Message-ID: <20140110165041.GH7572@laptop.programming.kicks-ass.net>
Date:	Fri, 10 Jan 2014 17:50:41 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH v2 1/1] mm: fix the theoretical compound_lock() vs
 prep_new_page() race

On Fri, Jan 10, 2014 at 05:12:27PM +0100, Oleg Nesterov wrote:
> The recent "[PATCH v6 tip/core/locking 3/8] Documentation/memory-barriers.txt:
> Prohibit speculative writes" from Paul says:
> 
> 	No SMP architecture currently supporting Linux allows speculative writes,
> 
> 	...
> 
> 	+ACCESS_ONCE(), which preserves the ordering between
> 	+the load from variable 'a' and the store to variable 'b':
> 	+
> 	+       q = ACCESS_ONCE(a);
> 	+       if (q) {
> 	+               ACCESS_ONCE(b) = p;
> 	+               do_something();
> 	+       }
> 
> 
> We can't use ACCESS_ONCE(), but I think that
> 
> 		if (PageTail(page)) {
> 			barrier();
> 			compound_lock(page_head);
> 		}
> 
> should obviously work (even if compound_lock() didn't imply mb).

The compiler can actually screw you over if that's preceded by something
like: SetPageTail(page). In which case it can prove that if (PageTail())
is a non-condition.

But yes, barring that, the version with barrier() in should stop the
compiler from doing most terrible things and it ought to work out.
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