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Mon, 23 Jan 2023 00:02:49 -0800 (PST) Message-ID: <4d26df97-3725-182b-6312-fa5cd8e9f85d@gmail.com> Date: Mon, 23 Jan 2023 10:02:47 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.7.0 Subject: Re: [PATCH v6 2/5] lazy tlb: allow lazy tlb mm refcounting to be configurable Content-Language: en-US From: Nadav Amit To: Nicholas Piggin , Andrew Morton Cc: Andy Lutomirski , Linus Torvalds , linux-arch , linux-mm , linuxppc-dev@lists.ozlabs.org References: <20230118080011.2258375-1-npiggin@gmail.com> <20230118080011.2258375-3-npiggin@gmail.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam03 X-Stat-Signature: khrjeab3khsaq8e5qyk7h4cg5swwki61 X-Rspamd-Queue-Id: CA8ABC0010 X-HE-Tag: 1674460971-946715 X-HE-Meta: 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 q/lGEVQO eIs3J6rJIoaZiCahglP605cbTCeoHdRDV/bsQGJBOjRAKXyZOIkJ/+Vl94RCo5h9uiU956rceRM694BNWIjYqpboqNAFdpoKQr7zKMIzC0NplV63x8+5qYfYDujoTic6jpETkA+lanOZuR5pfp77AkXlv/gDz8ciMKg5vBmg0rx/rbkpql1KCJobbKvWGPozo0aXZJbtNzXFEXDx/sgmWMP1KJMn7OZP0+t2dcJBb+4UiAdiaYgWN5bZsojxn6x0rwaSwwttkMqn0jWT/SxAQqjlY0/2ylB2QTqoJP8KH78BL1nAvHTn3mXhwlnJjiB0OXH8Q8rbIrZM4cFa4hwlRqbhmr+IhHkgIf9wiUaGrHxRvdd/aPZaMNC0BOSkLOFvbN9pV4b+pGyeHK+D207uEBRD0A67ZZMtXLtqIpFMxauRMeiVB2ezLLk91W3Zg68ZqlshVfr5N0nV2sIZksbv44ICN8ImUaXsdP1fR 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 1/23/23 9:35 AM, Nadav Amit wrote: >> +    if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_REFCOUNT)) { >> +        mmdrop(mm); >> +    } else { >> +        /* >> +         * mmdrop_lazy_tlb must provide a full memory barrier, see the >> +         * membarrier comment finish_task_switch which relies on this. >> +         */ >> +        smp_mb(); >> +    } >>   } > > Considering the fact that mmdrop_lazy_tlb() replaced mmdrop() in various > locations in which smp_mb() was not required, this comment might be > confusing. IOW, for the cases in most cases where mmdrop_lazy_tlb() > replaced mmdrop(), this comment was irrelevant, and therefore it now > becomes confusing. > > I am not sure the include the smp_mb() here instead of "open-coding" it > helps. I think that I now understand why you do need the smp_mb() here, so ignore my comment.