On Tue, 10 Jun 2014, Vlastimil Babka wrote: > > I think the compiler is allowed to turn this into > > > > if (ACCESS_ONCE(page_private(page)) > 0 && > > ACCESS_ONCE(page_private(page)) < MAX_ORDER) > > low_pfn += (1UL << ACCESS_ONCE(page_private(page))) - 1; > > > > since the inline function has a return value of unsigned long but gcc may > > not do this. I think > > > > /* > > * Big fat comment describing why we're using ACCESS_ONCE(), that > > * we're ok to race, and that this is meaningful only because of > > * the previous PageBuddy() check. > > */ > > unsigned long pageblock_order = ACCESS_ONCE(page_private(page)); > > > > is better. > > I've talked about it with a gcc guy and (although he didn't actually see the > code so it might be due to me not explaining it perfectly), the compiler will > inline page_order_unsafe() so that there's effectively. > > unsigned long freepage_order = ACCESS_ONCE(page_private(page)); > > and now it cannot just replace all freepage_order occurences with new > page_private() accesses. So thanks to the inlining, the volatile qualification > propagates to where it matters. It makes sense to me, but if it's according to > standard or gcc specific, I don't know. > I hate to belabor this point, but I think gcc does treat it differently. If you look at the assembly comparing your patch to if you do unsigned long freepage_order = ACCESS_ONCE(page_private(page)); instead, then if you enable annotation you'll see that gcc treats the store as page_x->D.y.private in your patch vs. MEM[(volatile long unsigned int *)page_x + 48B] with the above. I don't have the ability to prove that all versions of gcc optimization will not choose to reaccess page_private(page) here, but it does show that at least gcc 4.6.3 does not consider them to be equivalents. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>