From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3ED683624CC for ; Fri, 26 Jun 2026 08:17:22 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782461844; cv=none; b=X+H/eYWWffQbmcT+ZWsFP3iTKLxFijCBzLXazy9Q9Kr/uA0D09L7aYleLmTpb1n5TRRHJUGzEVI/iK5mWRz4+qtuDbUapQ/v0I6+Je1mOrVyJFeeuycR/0sTHE5lRpxrBna/mWRgCrrtWTU8XrTVlEGYFufMxqJknpxmYAD/1Zc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782461844; c=relaxed/simple; bh=IYv34gbtCWaj2pPgaOA9s0EPj21phP0d/rH6QtUfn08=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=dFzUsBmGIUJP+sER2nNaqeMY9A8/Xpj1Iw5wCm6W33ZlUWEwpmf6KLdf4XGaEWHhieiKuDANhiDIlNC+WSgraY/UzsvBV8hvr4VqJC/h27gzk24IwydqsJgzxY+ucSwNrAFcSCLw3m7Yr5VWU+4c0kg3pdAk8QXjdYZ6+ArWJ1Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=HRPKxo0R; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="HRPKxo0R" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0CB4E1F000E9; Fri, 26 Jun 2026 08:17:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782461842; bh=/AnaYuc4P67ER2npVfM1Eb7butqeY9jqtcroJ08g3DQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=HRPKxo0Rr3pbVCWT//XwS+W4dEPHI6hqcz1BIoaq/8A0qT653i2ccP8D1Qww3C6ky ZHc7iEGKxZLeGyTypJxFtRlnjb/2M3CM0cO28M2puxzKI5v4P4mt6TjFKzFE4BuSSP 3IO6sBs03lpsURbOoVFbQFXSE4G53T0ZCSQqMnfI7+P433zwNCUWHBb0QTdHYJe35W YayvavbbDgufFAFMzARoxcEMsGrUUxi2pIUkGdpv6gts0HDwuhPomEWmife+pLGAB2 i13ajRFu0ZeqIkoJ4iqrcwwv4xJ8mOBrtEYhSQCsxbEIHTCi5bD/4ggAtjwxoJqcPR 5Q8Yh24sykFlQ== Message-ID: <04ea3e05-5b83-4c2d-8759-67c544a7b484@kernel.org> Date: Fri, 26 Jun 2026 10:17:14 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] mm: annotate data-race in cpu_needs_drain() To: Xuewen Wang , akpm@linux-foundation.org, liam@infradead.org, ljs@kernel.org, vbabka@kernel.org, jannh@google.com, pfalcato@suse.de, chrisl@kernel.org, kasong@tencent.com, shikemeng@huaweicloud.com, nphamcs@gmail.com, baoquan.he@linux.dev, baohua@kernel.org, youngjun.park@lge.com, qi.zheng@linux.dev, shakeel.butt@linux.dev, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260626053700.2036899-1-wangxuewen@kylinos.cn> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/26/26 07:37, Xuewen Wang wrote: > KCSAN reports a data-race when cpu_needs_drain() reads another CPU's > per-cpu folio_batch->nr without locking, while the owning CPU writes > to it via folio_batch_add(). > > Reading a slightly stale value is harmless -- cpu_needs_drain() only > decides whether to schedule a drain, and the next iteration of > __lru_add_drain_all() will re-check. Use data_race() to annotate > the intentional race. > > Signed-off-by: Xuewen Wang > --- > Changes in v3: > - Wrap the entire || expression in a single data_race() instead of wrapping > each folio_batch_count() call individually, as suggested by Pedro and Lorenzo. > This is equally effective and more readable. > - Remove data_race() from need_mlock_drain(), as it is now covered by the data_race() > in cpu_needs_drain(). > v2: > https://lore.kernel.org/all/20260625065153.1581419-1-wangxuewen@kylinos.cn/ > v1: > https://lore.kernel.org/all/20260624092606.1083449-1-wangxuewen@kylinos.cn/ > --- > mm/swap.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/mm/swap.c b/mm/swap.c > index 588f50d8f1a8..46ea207e0624 100644 > --- a/mm/swap.c > +++ b/mm/swap.c > @@ -828,13 +828,13 @@ static bool cpu_needs_drain(unsigned int cpu) > struct cpu_fbatches *fbatches = &per_cpu(cpu_fbatches, cpu); > > /* Check these in order of likelihood that they're not zero */ > - return folio_batch_count(&fbatches->lru_add) || > + return data_race(folio_batch_count(&fbatches->lru_add) || > folio_batch_count(&fbatches->lru_move_tail) || > folio_batch_count(&fbatches->lru_deactivate_file) || > folio_batch_count(&fbatches->lru_deactivate) || > folio_batch_count(&fbatches->lru_lazyfree) || > folio_batch_count(&fbatches->lru_activate) || > - need_mlock_drain(cpu) || > + need_mlock_drain(cpu)) || The indentation is a bit suboptimal now. Would read nicer as diff --git a/mm/swap.c b/mm/swap.c index 588f50d8f1a8c..5958e6fdd3593 100644 --- a/mm/swap.c +++ b/mm/swap.c @@ -828,13 +828,13 @@ static bool cpu_needs_drain(unsigned int cpu) struct cpu_fbatches *fbatches = &per_cpu(cpu_fbatches, cpu); /* Check these in order of likelihood that they're not zero */ - return folio_batch_count(&fbatches->lru_add) || - folio_batch_count(&fbatches->lru_move_tail) || - folio_batch_count(&fbatches->lru_deactivate_file) || - folio_batch_count(&fbatches->lru_deactivate) || - folio_batch_count(&fbatches->lru_lazyfree) || - folio_batch_count(&fbatches->lru_activate) || - need_mlock_drain(cpu) || + return data_race(folio_batch_count(&fbatches->lru_add) || + folio_batch_count(&fbatches->lru_move_tail) || + folio_batch_count(&fbatches->lru_deactivate_file) || + folio_batch_count(&fbatches->lru_deactivate) || + folio_batch_count(&fbatches->lru_lazyfree) || + folio_batch_count(&fbatches->lru_activate) || + need_mlock_drain(cpu)) || has_bh_in_lru(cpu, NULL); } But I'll let others decide :) Acked-by: David Hildenbrand (Arm) -- Cheers, David