From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 385B5E63CB7 for ; Mon, 26 Jan 2026 00:33:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Type:Cc:To:From: Subject:Message-ID:References:Mime-Version:In-Reply-To:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=YzvRauRfmowbTuXqteNercyqQMxwj4TFlVBC4P/9vK8=; b=3cY/mnRCfD+AaU5SGb8TO1yZQp +zclAwqOMgkh9RxRHBoHAwiGgW7zLob4TZLlJLYcn/YL0M4YEcTLN6tSgosa8dvIXscHymnYumlZL iF2rqvYI0zc0d95AsRfvdAWsBzN99YpJbVQXENW3eyzpmJ9f8hLgCpJ9gdIgTK0vKkM7l5sqkilZH MWFCoC17wKd1Z1w1NyJX+3U6c8QddSL782du0oWMIbxKoIt1uJ98FQMRNqqHuThvY/89t1e5x8Y7d G4NsO/BVOdYXhvNbZB65optTJ///Jl05cxaSy1XRuSqp3U6j3LIdy3pVk55O39qMfGLTZmkpo/4td rQz4WqNA==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vkAXe-0000000BiPH-0a7n; Mon, 26 Jan 2026 00:33:18 +0000 Received: from mail-wm1-x349.google.com ([2a00:1450:4864:20::349]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1vkAXb-0000000BiOG-1OzV for linux-arm-kernel@lists.infradead.org; Mon, 26 Jan 2026 00:33:16 +0000 Received: by mail-wm1-x349.google.com with SMTP id 5b1f17b1804b1-4803b4e3b9eso31726795e9.3 for ; Sun, 25 Jan 2026 16:33:14 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1769387593; x=1769992393; darn=lists.infradead.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=YzvRauRfmowbTuXqteNercyqQMxwj4TFlVBC4P/9vK8=; b=V722QcCJ2yqEroy/alDZTxh3q/WlgMIiMWPf9s/KEPmNcHiX6emHJM1ij+eA3mKXJC lJnH3YW+0bKprtcu3FJuBy3S5axfgX7iXn05DsHoe6IjfX9dHiYD4uFfUH2vIOCJyk14 v29ByR5ufXi+sCMiUVsXrwc/xHDt5TLqJWUvdWpPkCJHZKV8fCkQu4srG7Evv+llYtmA yvjIxo8rNEB+Sg2KQlIIdgnGS2o3hjlXMmni8ljyOm+8ceUmiCSgd1aYZ9bAKRy2r2QB UO7r9jHCDh2/9aJUkmCcw9LFUjyb+TMyFLEjwZFD0LTtM6LehkhL2fjdGj9c4bbrud1x dyCw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1769387593; x=1769992393; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=YzvRauRfmowbTuXqteNercyqQMxwj4TFlVBC4P/9vK8=; b=Yic2+ZdjDjVdTLOuR7yX3bJh4wXq5+sEH8UNbB1w+ZiYezwm8au5kTdzwy5YZSeMx9 9eR9NcezqlpngKYR5DTQEkkzuc5qNF43xun7JZgUxZMKiDq4XiZgUyTIsSINro9Vk9QG 21ysqUb6BaL4tXOmV40oPfIYXeHyYX0kwa7x+gi+N9qKDtLyFKT7HwRrtT5TXbDXgEFg aFMv7ZwTzGxuY9hzctoEkRaJojOmF/cHGo/RZT+puj9LDlre5DEyq0ydw65SROhqXzuv eW8nWKaRlsuCpDekV2mRqKggbClgEySKW2IDE5o0luZIGTDaPVp6NdJ4jM8DZKrRTsn/ OOyg== X-Forwarded-Encrypted: i=1; AJvYcCWR6GtjgltssLLSuPbtXPiWj99IqLxwW7wcvbrpDopOId/IMNMAi8tKvlRWwytgQ4CG8Yl4isRhUmqRzR8yocUq@lists.infradead.org X-Gm-Message-State: AOJu0Yw1iS40fRsiw2ljIdok+MPJG304r8/6TCGHHE96TFgbfC+WXmEY 8crDnBVJnJvlF1su6NYRT1YalblirEMPHoXu8AAukSethoAw13VKDXZ0KcvJxxxIMz9ihvVmeZF rYQ== X-Received: from wmps41.prod.google.com ([2002:a05:600c:31a9:b0:480:4a03:7b7d]) (user=elver job=prod-delivery.src-stubby-dispatcher) by 2002:a05:600c:4684:b0:47e:e20e:bba3 with SMTP id 5b1f17b1804b1-4805cd44c41mr51919685e9.7.1769387593066; Sun, 25 Jan 2026 16:33:13 -0800 (PST) Date: Mon, 26 Jan 2026 01:25:12 +0100 In-Reply-To: <20260126002936.2676435-1-elver@google.com> Mime-Version: 1.0 References: <20260126002936.2676435-1-elver@google.com> X-Mailer: git-send-email 2.52.0.457.g6b5491de43-goog Message-ID: <20260126002936.2676435-4-elver@google.com> Subject: [PATCH 3/3] arm64, compiler-context-analysis: Permit alias analysis through __READ_ONCE() with CONFIG_LTO=y From: Marco Elver To: elver@google.com, Peter Zijlstra , Will Deacon Cc: Ingo Molnar , Thomas Gleixner , Boqun Feng , Waiman Long , Bart Van Assche , llvm@lists.linux.dev, Catalin Marinas , Arnd Bergmann , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kernel test robot Content-Type: text/plain; charset="UTF-8" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260125_163315_408982_5501DFE5 X-CRM114-Status: GOOD ( 17.63 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org When enabling Clang's Context Analysis (aka. Thread Safety Analysis) on kernel/futex/core.o (see Peter's changes at [1]), in arm64 LTO builds we could see: | kernel/futex/core.c:982:1: warning: spinlock 'atomic ? __u.__val : q->lock_ptr' is still held at the end of function [-Wthread-safety-analysis] | 982 | } | | ^ | kernel/futex/core.c:976:2: note: spinlock acquired here | 976 | spin_lock(lock_ptr); | | ^ | kernel/futex/core.c:982:1: warning: expecting spinlock 'q->lock_ptr' to be held at the end of function [-Wthread-safety-analysis] | 982 | } | | ^ | kernel/futex/core.c:966:6: note: spinlock acquired here | 966 | void futex_q_lockptr_lock(struct futex_q *q) | | ^ | 2 warnings generated. Where we have: extern void futex_q_lockptr_lock(struct futex_q *q) __acquires(q->lock_ptr); .. void futex_q_lockptr_lock(struct futex_q *q) { spinlock_t *lock_ptr; /* * See futex_unqueue() why lock_ptr can change. */ guard(rcu)(); retry: >> lock_ptr = READ_ONCE(q->lock_ptr); spin_lock(lock_ptr); ... } The READ_ONCE() above is expanded to arm64's LTO __READ_ONCE(). Here, Clang Thread Safety Analysis's alias analysis resolves 'lock_ptr' to 'atomic ? __u.__val : q->lock_ptr', and considers this the identity of the context lock given it can't see through the inline assembly; however, we simply want 'q->lock_ptr' as the canonical context lock. While for code generation the compiler simplified to __u.__val for pointers (8 byte case -> atomic), TSA's analysis (a) happens much earlier on the AST, and (b) would be the wrong deduction. Now that we've gotten rid of the 'atomic' ternary comparison, we can return '__u.__val' through a pointer that we initialize with '&x', but then change with a pointer-to-pointer. When READ_ONCE()'ing a context lock pointer, TSA's alias analysis does not invalidate the initial alias when updated through the pointer-to-pointer, and we make it effectively "see through" the __READ_ONCE(). Code generation is unchanged. Link: https://lkml.kernel.org/r/20260121110704.221498346@infradead.org [1] Reported-by: kernel test robot Closes: https://lore.kernel.org/oe-kbuild-all/202601221040.TeM0ihff-lkp@intel.com/ Cc: Peter Zijlstra Signed-off-by: Marco Elver --- arch/arm64/include/asm/rwonce.h | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h index 9963948f4b44..85b1dd7b0274 100644 --- a/arch/arm64/include/asm/rwonce.h +++ b/arch/arm64/include/asm/rwonce.h @@ -31,8 +31,10 @@ */ #define __READ_ONCE(x) \ ({ \ - typeof(&(x)) __x = &(x); \ + auto __x = &(x); \ + auto __ret = (TYPEOF_UNQUAL(*__x) *)__x, *__retp = &__ret; \ union { TYPEOF_UNQUAL(*__x) __val; char __c[1]; } __u; \ + *__retp = &__u.__val; \ switch (sizeof(x)) { \ case 1: \ asm volatile(__LOAD_RCPC(b, %w0, %1) \ @@ -57,7 +59,7 @@ default: \ __u.__val = *(volatile typeof(*__x) *)__x; \ } \ - __u.__val; \ + *__ret; \ }) #endif /* !BUILD_VDSO */ -- 2.52.0.457.g6b5491de43-goog