From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-wm1-f74.google.com (mail-wm1-f74.google.com [209.85.128.74]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 796F519DF4D for ; Mon, 26 Jan 2026 00:33:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.128.74 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769387595; cv=none; b=jBrIfXFqnalsY6JiM4vgeN4fPaPubdwCPq07erJBJ+wvxZzK1Ps/8zT5W+g8jNNLzW2LwEG7xeU6jI0tuKSikKfuyZxEoPH/BUuIwffI3BGLcFFonlbeu3RFBAYQZUbKYNtbE+XzBrdt7w1TO+ortjr788V9HygB5NZfxgKdzA4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769387595; c=relaxed/simple; bh=fyabtr/WWe4ln44fb6JDEbFXLec2d5CvMKWPch2hh8I=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=g+l5JISn73BC763hUakdluINKo1t9y5Ppu1FAH+N1zpQ9IKM/cKhIy6dkpbhX8CECRNyEWECMUyl92J06XbWgOWDTX24SXLPIVmGxtum3UePgbVwETxrwykvEL/Uwqmh7pP1QM4LBLE3u+TPDEUc8O4xfBXraAitzkT4O+BmB0k= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--elver.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=2C/8hKRN; arc=none smtp.client-ip=209.85.128.74 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--elver.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="2C/8hKRN" Received: by mail-wm1-f74.google.com with SMTP id 5b1f17b1804b1-4803b4e3b9eso31726765e9.3 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.linux.dev; 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=2C/8hKRNOaXgChhSc3nOQs0M5ruuEJnfzCJ16TL3bFwrCzyJJ1Oct4MYzjVjGb6MzF CfhpqjqnKKJQefmx4np/90jzggXfrZUgN1EG5KF0DCtljgdQWtYyoJnfM0iuJ1r5U8jh mwO7SKhm64X8aiK06ATJgwtJ3YZ4stK9AhNVzcd6JLtA8HWrFT0Xo/UCH4FX6P6DNoRo VPnuflzeaXy3c68Mxc5TQnNMFFzkGXJednipRonC1TldEinaxTYBcAAx/PujwxnpGhFr buTj93JpC561v0aJTwsVAKZeW0WwyvZm6dNcD6rQbVJbBZJ+GkfFlEQsE/d8P2ypP0zR keYQ== 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=MWVHYORsfWc2thWThdz7rmJkPOt0rQovSRrcJRRZ69/cU4GNA1v7Ox0H71+NBWLSEN LXOsrB/TI/J4qGEsYuN/Caoumzge/z2QIMPUMPO+hXd9jiQURynd8RYTNt7KbYUpToYA rtnVDrMmfvweEUbR1mhfWKe9V7TnUIya1xZCW6NZSXuctxVbx/k+dwXl/K/IYQmkFJY2 TLHOFSd6S4Amjrq+M/e1t+R4w0txo8NNyjHGpERLy9XkudlaUghioFR6rRCZgsc7dsUi qC5NO691VnmIIcOiZVOAMJeiNlYBERaRHtnjv2hFR+wo4CJXbIAyXEI2yhn7gEZzL933 QGDw== X-Forwarded-Encrypted: i=1; AJvYcCVuGqNRLRfWy6VjppzfzAGzSU1AZ0f8mQmfS3ZadGAme04pL/8UosjoBnQN1RW7kBuJRygD@lists.linux.dev X-Gm-Message-State: AOJu0Yy+SozAON5xwT3IVHAH8JHfSPg8RvRw4b31Yt2QCmGapbix6VJA 0mPK4dzgwNCZmsZdphDCCZIMOFXYyKqeKGONADYg6FY6nbTA+N+tL0nmPMl+kr7vp4Mg5kEJjAL tWA== 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> Precedence: bulk X-Mailing-List: llvm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: 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" 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