From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 3B72B3A6B88; Fri, 13 Mar 2026 14:42:24 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773412945; cv=none; b=AR0rp8lMPmCt+g/8SyN400w0aOTZVSpRco2MfVf2FFbt2+KVLO7s8mW1gnrJYSpivhFjKt8FxAt1UEEUpbINM9GmwphAxZhPfC5kTu2HrLdX2DB9eEcpRkKX8XmgMYy/msCaScxvLqdSPHreB6TNZaf6uHkHx4Gb1ZSz/5dMv6E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773412945; c=relaxed/simple; bh=9amcathOnSuA9o0wdffWhIzxuO1Tcw+QsTIsAnvrXUw=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Tp8IKkIKbNjQpvY0R3b5M0Sl+dChRtjRX1Bve024XLkQ4NJZTB8YWzNATX5gE8qKDV8HgGpgGu+vD4nkWN84EMY6Z0on7z3YEa4nhyc98R982phrQFZ0KfWfp8BiwyKtpHYoR//fhJYd6mE1BGqwB/Clgbk2ygVMR4Er3pC1Ma8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 87823165C; Fri, 13 Mar 2026 07:42:17 -0700 (PDT) Received: from arm.com (usa-sjc-mx-foss1.foss.arm.com [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 3B60C3F7BD; Fri, 13 Mar 2026 07:42:21 -0700 (PDT) Date: Fri, 13 Mar 2026 14:42:18 +0000 From: Catalin Marinas To: Yeoreum Yun Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kvmarm@lists.linux.dev, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org, will@kernel.org, maz@kernel.org, oupton@kernel.org, miko.lenczewski@arm.com, kevin.brodsky@arm.com, broonie@kernel.org, ardb@kernel.org, suzuki.poulose@arm.com, lpieralisi@kernel.org, joey.gouly@arm.com, yuzenghui@huawei.com Subject: Re: [PATCH v15 5/8] arm64: futex: support futex with FEAT_LSUI Message-ID: References: <20260227151705.1275328-1-yeoreum.yun@arm.com> <20260227151705.1275328-6-yeoreum.yun@arm.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Fri, Mar 13, 2026 at 09:23:58AM +0000, Yeoreum Yun wrote: > > On Fri, Feb 27, 2026 at 03:17:02PM +0000, Yeoreum Yun wrote: > > > + > > > + if (__lsui_cmpxchg64(uaddr64, &oval64.raw, nval64.raw)) > > > + return -EFAULT; > > > + > > > + oldval = oval64.futex[futex_pos]; > > > + other = oval64.futex[other_pos]; > > > + orig_other = orig64.futex[other_pos]; > > > + > > > + if (other == orig_other) { > > > + ret = 0; > > > + break; > > > + } > > > > Is this check correct? What if the cmpxchg64 failed because futex_pos > > was changed but other_pos remained the same, it will just report success > > here. You need to compare the full 64-bit value to ensure the cmpxchg64 > > succeeded. > > This is not matter since "futex_cmpxchg_value_locked()" checks > the "curval" and "oldval" IOW, though it returns success, > caller of this function always checks the "curval" and "oldval" > and when it's different, It handles to change return as -EAGAIN. Ah, ok, it makes sense (I did not check the callers). > > > + } > > > + > > > + if (!ret) > > > + *oval = oldval; > > > + > > > + return ret; > > > +} > > > + > > > +static __always_inline int > > > +__lsui_futex_atomic_and(int oparg, u32 __user *uaddr, int *oval) > > > +{ > > > + /* > > > + * Undo the bitwise negation applied to the oparg passed from > > > + * arch_futex_atomic_op_inuser() with FUTEX_OP_ANDN. > > > + */ > > > + return __lsui_futex_atomic_andnot(~oparg, uaddr, oval); > > > +} > > > + > > > +static __always_inline int > > > +__lsui_futex_atomic_eor(int oparg, u32 __user *uaddr, int *oval) > > > +{ > > > + u32 oldval, newval, val; > > > + int ret, i; > > > + > > > + if (get_user(oldval, uaddr)) > > > + return -EFAULT; > > > + > > > + /* > > > + * there are no ldteor/stteor instructions... > > > + */ > > > + for (i = 0; i < FUTEX_MAX_LOOPS; i++) { > > > + newval = oldval ^ oparg; > > > + > > > + ret = __lsui_cmpxchg32(uaddr, oldval, newval, &val); > > > > Since we have a FUTEX_MAX_LOOPS here, do we need it in cmpxchg32 as > > well? > > > > For eor, we need a loop irrespective of whether futex_pos or other_pos > > have changed. For cmpxchg, we need the loop only if other_pos has > > changed and return -EAGAIN if futex_pos has changed since the caller > > needs to update oldval and call again. > > > > So try to differentiate these cases, maybe only keep the loop outside > > cmpxchg32 (I haven't put much though into it). > > I think we can remove loops on __lsui_cmpxchg32() and return -EAGAIN > when other_pos is different. the __lsui_cmpxchg32() will be called > "futex_cmpxchg_value_locked()" and as I said, this always checks > whether curval & oldval when it successed. Yes, I think for the futex_cmpxchg_value_locked(), the bounded loop doesn't matter since the core would invoke it back on -EAGAIN. It's nice not to fail if the actual futex did not change but in practice it doesn't make any difference and I'd rather keep the code simple. > But in "eor" when it receive "-EAGAIN" from __lsui_cmxchg32() > we can simply continue the loop. Yes, for eor we need the bounded loop. Only return -EAGAIN to the user if we finished the loop and either __lsui_cmpxchg32() returned -EAGAIN or the updated on futex_pos failed. Thanks. -- Catalin