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Wed, 09 Sep 2026 14:29:58 -0700 (PDT) From: Guilherme Giacomo Simoes To: ljs@kernel.org Cc: akpm@linux-foundation.org, david@kernel.org, lance.yang@linux.dev, liam@infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mhocko@suse.com, pfalcato@suse.de, rppt@kernel.org, surenb@google.com, trintaeoitogc@gmail.com, vbabka@kernel.org, willy@infradead.org Subject: Re: [PATCH] mm: bypass datarace check Date: Wed, 9 Sep 2026 18:29:43 -0300 Message-ID: <20260909212943.539665-1-trintaeoitogc@gmail.com> X-Mailer: git-send-email 2.52.0 In-Reply-To: References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: A3709140002 X-Stat-Signature: sctg94kja14zpifzshfxftmnrkemuytr X-HE-Tag: 1788989399-662254 X-HE-Meta: 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 A8+ELJbg fDV8TmINNH/IyQHrqbSQgvAz9bNRAptR5AehZKE1K/ZngZaeZROz63PELX0KSIdkptSQEp8dou1DSeiZfoJUcCznfL8J51UFUetD0X4u8bn2urSK5UsZLPzVVt1X/uoCjF47iOhVeRVmqgBdx8ShjuxUFYHdz/p+u3JJ9JVWjeaJf3L3zzSOHGkLmHIpApcNnOR74Uzko0JlfvJIAP5w3QDodOweaKUxV4VN6eQuZJUrdNme6mYMzpSufFlzMjDUalCxkBe4gHZBYtDMyJMldzP/miZullVPYcX3J8+relSi3puNFKxvO29LmLYthocLNA9nyRcHlkJe73VxC3wejWkAL8kWkn3C40b48G/gIQHYrYfO0mQ3Rpj2Rdio2ac9p1rF5d7nyofjmxJFVdrdt87CSEqoiaQYreCeE0mZvSC3T2FH1aD/1Fm6Wmg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: "Lorenzo Stoakes (ARM)" wrote: > I started review below but honestly this patch is confused in multiple ways > and it's not entirely clear you really understand what's going on here. I can be wrong, but was understand that due the order that the code was write probably the data race problem will not happen. The reader (__vmf_anon_prepare()): ``` if (likely(vma->anon_vma)) // lockless check return 0; // OK // if the check above fail if (!__anon_vma_prepare(vma)) // called the __anon_vma_prepare return 0; ``` inside __anon_vma_prepare() ``` spin_lock(&mm->page_table_lock); //ACQUIRE semantics if (likely(!vma->anon_vma)) // re-check under lock // ... alloc all spin_unlock(&mm->page_table_lock); ``` This is safe because, if `if (likely(vma->anon_vma))` return NULL, we will got the mmap_lock and then page_table_lock. The critical re-check inside __anon_vma_prepare() happens under spin_lock(...) with has ACQUIRE semantics. With ACQUIRE semantics , the cpu (or compiler, I don't know) cannot reorder the memory access acress the lock boundary. I'm right? > > It's also basically implementing what we suggested. > > So at this point I think it's easier if I send the patch with a: > > Reported-by: > Closes: > > tag -> you, this patch. > > Thanks! ok, no problem > On Wed, Sep 09, 2026 at 08:57:23AM -0300, Guilherme Giacomo Simoes wrote: > > Despiste kcsan point to a possible race condition problem, this is a > > Typos -> Despite, point -> points Hmm, is not the first time that any person points my english mistakes... I will improve this point, thank you for yout jints > > safe race condition due the access memory ordering, since > > spin_lock(&mm->page_table_lock) have ACQUIRE semantics and ensure the > > ordering mapping. > > This sentence is a bit confused. Acquire semantics mean absolutely nothing > unless paired with another operation and etc. etc. missing full stop, my bad. > Needs a: > > Suggested-by: Pedro Falcato Yeah, I forget > > Also: > > Assisted-by: LLM? > The list below reads very LLM-ish so I have to ask did you use one etc. etc. > > https://docs.kernel.org/process/coding-assistants.html > > Perhaps given I am suggesting a lot here a: I don't have installed any llm (not even cursor), I just use a deepseek, chatgpt, etc.. to clear up a few questions. (maybe I should start use this to help me with english too) > There are other places where this check is done and etc. I would should checked this, sorry. Anxiety. Thanks Lorenzo for your review, help and patience