Re: bit fields && data tearing

Peter Zijlstra
2014-09-05T19:52:34Z

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.