Re: BUG: Global FPU corruption in 2.2

[Alan Cox]

> >         child->flags |= PF_PTRACED; 
> > 
> > without waiting for the child to have stopped. 
> 
> I can see how this could case PF_USEDFPU to be cleared inadvertently,
> but I do not have any ideas for testing this.  Is it clear that this
> is the source of the problem?

There is no guarantee that |= is implemented atomically - in fact its quite
likely to read

		get child->flags
		or PF_PTRACED
		write child->flags

and a PF_USEDFPU on another processor at the same instant -would- end up being
lost.

There are two fixes

1.	Make all the ops atomic (foo_bit())
2.	Split the flags

The preferable one for performance is certainly to backport the 2.4 changes