From nobody Mon Feb 9 05:52:12 2026 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 796A013A258 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=1769387597; cv=none; b=tiRC7iAmcyPsSHgAzSFFybguhMzlbDbAIzRQaDuqfn5tYQKFD8okWhQiuBcWpaSag8Fs7WUcIbhb1C/bOnsfNH9TvLQ0FfLiiUUYTUfezUijrLkZKdvV7S7OaTrm1iOXBWe4blsbK/Wdh2aLUKe/GyXKwTDZ2sapyyQiKecDMms= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769387597; c=relaxed/simple; bh=fyabtr/WWe4ln44fb6JDEbFXLec2d5CvMKWPch2hh8I=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=rQYWJyXKLZ66Q3qK/Ph/wpVc2L1ZuQo6Abkimj+RmWxErlEsOvNNYuyWBY8MHCicO6ddav+jQKEP7EiM6QIVwFBn7m3QUGUJQSPbP6wt03emPvg9yn8UY9f3CDhWDEvVBgXtHVfyjxk8C3cpuQgzs0tfmBT+5x2JiDwK4j9Kuf4= 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=u/gGmr8P; 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="u/gGmr8P" Received: by mail-wm1-f74.google.com with SMTP id 5b1f17b1804b1-4801e2e3532so26681925e9.2 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=vger.kernel.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=u/gGmr8PNq6uIrGXBXui95cmtMYTnYk/sqzPJ/WI9uQPT8OjDxbSJlF1Wl2+DIo29C GOzJomOIMTY/jtLRxj5t1u1fQKRU+46Dux91RGW/mhr9EyLB3kKHcF9S4cXPUHBNe2TJ KMqY8nYHHo+UG7JA7QCYh83x1sld9SFG8Ypzpge8YPM0FayAAPClKkXEzSLGctfNrD0i I+ngfoNIfCCxMfeHakUEz/2SOuRPiTx6FKzAMnTFo/fiHtubDMNCkoQQiILpSLLLItLJ eEOKnZXQGjPafQ8SZz7/n//dHkFeVQt6Y6DwSpdfbeDJwUsPJwZ1J4umz66hTMI1Sld1 mRPQ== 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=dYUxfV/XYiWbeMi8rWli5obeQyOLtEcxXvxuU2+MJ7SApfu6bL+u1nBsQ8CW0QqSIM +/v772grNxt7iAxahtOsVjy6tivjN2GpkhqxHapxsWTG7YasW6V6hlF4+jEIV8QDXPOR m21mNd73k/mzYxVGg1TpZj61DvmL+sbBj35bIO2wY4kpepLyInfRhjyaSQKdIBhaRxEM 8d5b767RWBGwoyF+NJGTBRpaU60WLl7xGf3ytGFafRd7wq39fiZ4DZBV7fz+8UHekg2K t0CygHBp9XD92cSIOnlFIcGsndySOCaW6/mpkA74/ALukmJArQEIrb/hp7N/B1Gi3uS2 khMA== X-Forwarded-Encrypted: i=1; AJvYcCUGyqk79vK5vZ0Xvg3LqoXxAQU1Xo0yyYL29KM2nPHM1rGDgqxzo34eP9JBk1xiwiIMKXAtWCJ7mIcrFVk=@vger.kernel.org X-Gm-Message-State: AOJu0Yz0sYw6ltcQ+DYyOROSSn2d4Ds6CYSCcRM+XaelB3oWNId3TXq7 EuVKiBA0zeRj28OInoPZ/WdnU07ImdPae2/N88VuYvrn+e5Vt8cMagBYEMQhkMab7VB9nydHOKJ Tpg== 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: linux-kernel@vger.kernel.org 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-Transfer-Encoding: quoted-printable 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->loc= k_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 b= e 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 =3D 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@int= el.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/rwonc= e.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 =3D &(x); \ + auto __x =3D &(x); \ + auto __ret =3D (TYPEOF_UNQUAL(*__x) *)__x, *__retp =3D &__ret; \ union { TYPEOF_UNQUAL(*__x) __val; char __c[1]; } __u; \ + *__retp =3D &__u.__val; \ switch (sizeof(x)) { \ case 1: \ asm volatile(__LOAD_RCPC(b, %w0, %1) \ @@ -57,7 +59,7 @@ default: \ __u.__val =3D *(volatile typeof(*__x) *)__x; \ } \ - __u.__val; \ + *__ret; \ }) =20 #endif /* !BUILD_VDSO */ --=20 2.52.0.457.g6b5491de43-goog