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X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 11 May 2022 23:56:36.0777 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 878b5461-501f-42ac-208b-08da33a9e215 X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[12.22.5.238];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: CO1NAM11FT039.eop-nam11.prod.protection.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: MWHPR12MB1519 X-Rspamd-Queue-Id: C15172009F X-Stat-Signature: hfyyt51gzw1rpwzc5muzwdfjq414hpif Authentication-Results: imf03.hostedemail.com; dkim=pass header.d=Nvidia.com header.s=selector2 header.b=FRXlRYx+; dmarc=pass (policy=reject) header.from=nvidia.com; spf=none (imf03.hostedemail.com: domain of jhubbard@nvidia.com has no SPF policy when checking 40.107.93.68) smtp.mailfrom=jhubbard@nvidia.com X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1652313390-946090 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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, -- John Hubbard NVIDIA > In the end, it is your code, so you get to decide how much you would > like to keep track of what compilers get up to over time. ;-) > > Thanx, Paul