linux-kernel - Re: bit fields && data tearing

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Date:	Mon, 14 Jul 2014 08:25:24 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, Jakub Jelinek <jakub@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Richard Henderson <rth@...ddle.net>,
	Miroslav Franc <mfranc@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	Dan <opendtv@...oo.com>
Subject: Re: bit fields && data tearing

On Sun, 2014-07-13 at 09:15 -0400, Peter Hurley wrote:
> 
> I'm not sure I understand your point here, Ben.
> 
> Suppose that two different spinlocks are used independently to
> protect r-m-w access to adjacent data. In Oleg's example,
> suppose spinlock 1 is used for access to the bitfield and
> spinlock 2 is used for access to freeze_stop.
> 
> What would prevent an accidental write to freeze_stop from the
> kt_1 thread?

My point was to be weary of bitfields in general because access
to them is always R-M-W, never atomic and that seem to escape
people regularily :-) (Among other problems such as endian etc...)

As for Oleg's example, it *should* have worked because the bitfield and
the adjacent freeze_stop should have been accessed using load/stores
that don't actually overlap, but the compiler bug causes the bitfield
access to not properly use the basic type of the bitfield, but escalate
to a full 64-bit R-M-W instead, thus incorrectly R-M-W'ing the field
next door.

Cheers,
Ben.




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