bit fields && data tearing

[Paul E. McKenney]

On Fri, Sep 05, 2014 at 04:01:35PM -0400, Peter Hurley wrote:
> On 09/05/2014 03:52 PM, Peter Zijlstra wrote:
> > On Fri, Sep 05, 2014 at 11:31:09AM -0700, Paul E. McKenney wrote:
> >> compiler: Allow 1- and 2-byte smp_load_acquire() and smp_store_release()
> >>
> >> CPUs without single-byte and double-byte loads and stores place some
> >> "interesting" requirements on concurrent code.  For example (adapted
> >> from Peter Hurley's test code), suppose we have the following structure:
> >>     
> >>     	struct foo {
> >>     		spinlock_t lock1;
> >>     		spinlock_t lock2;
> >>     		char a; /* Protected by lock1. */
> >>     		char b; /* Protected by lock2. */
> >>     	};
> >>     	struct foo *foop;
> >>     
> >> Of course, it is common (and good) practice to place data protected
> >> by different locks in separate cache lines.  However, if the locks are
> >> rarely acquired (for example, only in rare error cases), and there are
> >> a great many instances of the data structure, then memory footprint can
> >> trump false-sharing concerns, so that it can be better to place them in
> >> the same cache cache line as above.
> >>
> >> But if the CPU does not support single-byte loads and stores, a store
> >> to foop->a will do a non-atomic read-modify-write operation on foop->b,
> >> which will come as a nasty surprise to someone holding foop->lock2.  So we
> >> now require CPUs to support single-byte and double-byte loads and stores.
> >> Therefore, this commit adjusts the definition of __native_word() to allow
> >> these sizes to be used by smp_load_acquire() and smp_store_release().
> > 
> > So does this patch depends on a patch that removes pre EV56 alpha
> > support? I'm all for removing that, but I need to see the patch merged
> > before we can do this.
> 
> I'm working on that but Alpha's Kconfig is not quite straightforward.
> 
> 
> ... and I'm wondering if I should _remove_ pre-EV56 configurations or
> move the default choice and produce a warning about unsupported Alpha
> CPUs instead?

I suspect that either would work, given that the Alpha community is
pretty close-knit.  Just setting the appropriate flag to make the
compiler generate one-byte and two-byte loads and stores would
probably suffice.  ;-)

							Thanx, Paul

> Regards,
> Peter Hurley
> 
> [ How does one do a red popup in kbuild?
>   The 'comment' approach is too subtle.
> ]
> 
> 
>