Re: bit fields && data tearing

From: Benjamin Herrenschmidt
Date: Sun Jul 13 2014 - 18:26:35 EST

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.


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