Re: bit fields && data tearing

From: Peter Hurley
Date: Tue Jul 15 2014 - 09:54:30 EST

On 07/13/2014 06:25 PM, Benjamin Herrenschmidt wrote:
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.

Yeah, ok, so just a generic heads-up about non-atomicity of bitfields,
and not something specific to Oleg's example. Thanks.

Jonathan Corbet wrote a LWN article about this back in 2012:

I guess it's fixed in gcc 4.8, but too bad there's not a workaround for
earlier compilers (akin to -fstrict_volatile_bitfields without requiring
the volatile keyword).

Peter Hurley

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