On Wed, May 11, 2022 at 05:12:32PM -0700, John Hubbard wrote: > On 5/11/22 16:57, John Hubbard wrote: > > On 5/11/22 16:45, Paul E. McKenney wrote: > > > > > > > > Well no, because the "&" operation is a single operation on the CPU, and > > > > isn't going to get split up like that. > > > > > > Chiming in a bit late... > > > > Much appreciated! > > > > > > > > The usual way that this sort of thing causes trouble is if there is a > > > single store instruction that changes the value from MIGRATE_ISOLATE > > > to MIGRATE_CMA, and if the compiler decides to fetch twice, AND twice, > > > > Doing an AND twice for "x & constant" this definitely blows my mind. Is > > nothing sacred? :) > > > > > and then combine the results. This could give a zero outcome where the > > > underlying variable never had the value zero. > > > > > > Is this sort of thing low probability? > > > > > > Definitely. > > > > > > Isn't this sort of thing prohibited? > > > > > > Definitely not. > > > > > > So what you have will likely work for at least a while longer, but it > > > is not guaranteed and it forces you to think a lot harder about what > > > the current implementations of the compiler can and cannot do to you. > > > > > > The following LWN article goes through some of the possible optimizations > > > (vandalisms?) in this area: https://lwn.net/Articles/793253/ > > > > > > > hmm, I don't think we hit any of those cases, do we? Because here, the > > "write" side is via a non-inline function that I just don't believe the > > compiler is allowed to call twice. Or is it? > > > > Minchan's earlier summary: > > > > CPU 0 CPU1 > > > > > > set_pageblock_migratetype(MIGRATE_ISOLATE) > > > > if (get_pageblock_migrate(page) & MIGRATE_CMA) > > > > set_pageblock_migratetype(MIGRATE_CMA) > > > > if (get_pageblock_migrate(page) & MIGRATE_ISOLATE) > > > > ...where set_pageblock_migratetype() is not inline. > > > > thanks, > > Let me try to say this more clearly: I don't think that the following > __READ_ONCE() statement can actually help anything, given that > get_pageblock_migratetype() is non-inlined: > > + int __mt = get_pageblock_migratetype(page); > + int mt = __READ_ONCE(__mt); > + > + if (mt & (MIGRATE_CMA | MIGRATE_ISOLATE)) > + return false; > > > Am I missing anything here? In the absence of future aggression from link-time optimizations (LTO), you are missing nothing. Thanx, Paul
