On 05/10/15 14:36, Jan Beulich wrote: >>>> On 05.10.15 at 15:18, <george.dun...@citrix.com> wrote: >> On 02/10/15 05:40, Juergen Gross wrote: >>> Use a bit mask for testing of a set bit instead of test_bit in case no >>> atomic operation is needed, as this will lead to smaller and more >>> effective code. >>> >>> Signed-off-by: Juergen Gross <jgr...@suse.com> >> >> I'm a bit confused here -- exactly when is an atomic operation needed or >> not needed? Isn't it the case that we always need to do an atomic read >> if we ever do an atomic write without a lock held? > > First of all - what is an atomic read from CPU perspective other than > just a read? Since we talk about individual bits here, we don't care > about the granularity the compiler may convert the read to, and even > if the compiler chose to do multiple reads the result would still be > correct, as only one of those reads can possibly have read the bit > in question. > > And then, the old mechanism was in no way "atomic", all it added was > a kind of compiler barrier (due to the cast to volatile). Yet in none of > the cases changed I was able to spot a need for such a barrier.
OK, so the key thing about test_bit isn't that it's atomic, so much that it's an implicit memory barrier. So as long as you're not doing a lockless careful-ordering sort of thing, then a simple memory read should be fine. Is that correct? In that case, it's likely that the patch is correct (though I'll take a closer look just to be sure). -George _______________________________________________ Xen-devel mailing list Xen-devel@lists.xen.org http://lists.xen.org/xen-devel
