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 B1F36E7C714 for ; Tue, 3 Feb 2026 11:47:18 +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:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From: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=+N2t80z6HSeiggWRybMdp8IMRjGvkmP9kSCLTCvwcNo=; b=pbvtL0SVdMoLzjsQ8e2yXMN1UW XHYfa0Iixt1/nZwWW/3PdYXYrja6MgtB9a9vz6n70T/tkMf8ak6jwXU1egoNEvNmIAo6pwzujRAK+ SDJamuH5sYM5klHhF+Eby3bX/Bwd1IH57OoKjg04wrXNxlBpsBLPj7P7qyyfXj8XsVwosqm3xe+vo naI7mWUUjs8tKTGzfcmXGx7PapJW3YUW6IPd4yP2yVMwxKuyafvJn0O+h+7fTmDRRG7B738YHWGqf kbixGRU6d7wMjnTNXEZWOv8rvksFmDQWLDzJ3oSkanZlGLSXb5Xfs1PiIaVBPTTDNjJzOaQ//vqF8 ZXTmWBcg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vnEsA-00000006Z01-1tyF; Tue, 03 Feb 2026 11:47:10 +0000 Received: from tor.source.kernel.org ([172.105.4.254]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1vnEs8-00000006Yzv-3jnj for linux-arm-kernel@lists.infradead.org; Tue, 03 Feb 2026 11:47:09 +0000 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by tor.source.kernel.org (Postfix) with ESMTP id 4035D601DF; Tue, 3 Feb 2026 11:47:08 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id D4823C2BCAF; Tue, 3 Feb 2026 11:47:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770119228; bh=ekFdSEhH50PU5wS7nUQFWI9pnatXudGXJZ1S7B3/w0Y=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=jlcEnHTzUfAtUvimkcZEXChZN4VWQ57oWH1OVySQzk35pmR0hEk+psTq2Z4MRsXfX prgWTBqYh2pe+/pmT8j8zIEdWk0AiM1+Kv6UYiM3mrGFtDDjRXIONbH9yreNp9EMOm A+T51hZXIBs5nFLdIeHBzRGMnodPiC3udI/PsXw2LIJjnIV6BIgZQAoini76m6qkIZ 5f8Xq0HGKrSAX1xaYpvUpUF3/9E7Jk+eZTXTD/QcaxJzxSxsa1tdqtElaHXWgdDYMO wnQ8MRjFHB5aIar5Ov3apPqP9zFphmqHk6LFXA6fW4wWGGScwiteefdX4T91skEUwj BT7zBR5Prd/DQ== Date: Tue, 3 Feb 2026 11:47:01 +0000 From: Will Deacon To: David Laight Cc: Marco Elver , Peter Zijlstra , 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 , Boqun Feng Subject: Re: [PATCH v3 3/3] arm64, compiler-context-analysis: Permit alias analysis through __READ_ONCE() with CONFIG_LTO=y Message-ID: References: <20260130132951.2714396-1-elver@google.com> <20260130132951.2714396-4-elver@google.com> <20260202192923.0707e463@pumpkin> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260202192923.0707e463@pumpkin> 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 On Mon, Feb 02, 2026 at 07:29:23PM +0000, David Laight wrote: > On Mon, 2 Feb 2026 15:39:36 +0000 > Will Deacon wrote: > > > On Fri, Jan 30, 2026 at 02:28:26PM +0100, Marco Elver wrote: > > > 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); > > > ... > > > } > > > > > > At the time of the above report (prior to removal of the 'atomic' flag), > > > Clang Thread Safety Analysis's alias analysis resolved 'lock_ptr' to > > > 'atomic ? __u.__val : q->lock_ptr' (now just '__u.__val'), and used > > > this as the identity of the context lock given it cannot "see through" > > > the inline assembly; however, we 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' was set), 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 update via 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 > > > Tested-by: Boqun Feng > > > Signed-off-by: Marco Elver > > > --- > > > v3: > > > * Use 'typeof(*__ret)'. > > > * Commit message. > > > > > > v2: > > > * Rebase. > > > --- > > > arch/arm64/include/asm/rwonce.h | 10 +++++++--- > > > 1 file changed, 7 insertions(+), 3 deletions(-) > > > > > > diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h > > > index 42c9e8429274..b7de74d4bf07 100644 > > > --- a/arch/arm64/include/asm/rwonce.h > > > +++ b/arch/arm64/include/asm/rwonce.h > > > @@ -45,8 +45,12 @@ > > > */ > > > #define __READ_ONCE(x) \ > > > ({ \ > > > - typeof(&(x)) __x = &(x); \ > > > - union { __rwonce_typeof_unqual(*__x) __val; char __c[1]; } __u; \ > > > + auto __x = &(x); \ > > > + auto __ret = (__rwonce_typeof_unqual(*__x) *)__x; \ > > > + /* Hides alias reassignment from Clang's -Wthread-safety. */ \ > > > + auto __retp = &__ret; \ > > > + union { typeof(*__ret) __val; char __c[1]; } __u; \ > > > + *__retp = &__u.__val; \ > > > switch (sizeof(x)) { \ > > > case 1: \ > > > asm volatile(__LOAD_RCPC(b, %w0, %1) \ > > > @@ -71,7 +75,7 @@ > > > default: \ > > > __u.__val = *(volatile typeof(*__x) *)__x; \ > > > } \ > > > - __u.__val; \ > > > + *__ret; \ > > > }) > > > > What does GCC do with this? :/ > > GCC currently doesn't see it, LTO is clang only. LTO is just one way that a compiler could end up breaking dependency chains, so I really want to maintain the option to enable this path for GCC in case we run into problems caused by other optimisations in future. Will