From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 36B3B47CA9D; Thu, 20 Aug 2026 16:23:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787242989; cv=none; b=rvttNhusJBk3QxJIDN1K2fufwJgZPrEMsC/ISqZQofOQHCxTosGln0tAGkKGaE1MTm0JZwIh/rpzyVMl9RFGUli446Lxoh50NFcmO+XMJmw0Mvypg1K7igQ3jO4o4d1kVoalzjZwhrOaqZviauXE4Lv77M/UM0vRUUZZUsMXR2I= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787242989; c=relaxed/simple; bh=D6cfcPHLXXXSlTNqSAg3JYV9wUq48i+PQgtNmzpSlc8=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=Cuf84degjcekDF34Yk5cwoenqFm8c+GC7EMEjZLDwehhQe5ILjzkZuTYY9z9nCIJKVGDeWsaqBdU7JigsGrl/1vDIePiitZWRZmlU8/XY+gzKRg4wzWULVeyIbagTBdcnEUgLUa7g7x2mHiOaZ95cxC5pm9wdDL8855JRnRcmFM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Nxv859dk; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Nxv859dk" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DA1A11F00A3A; Thu, 20 Aug 2026 16:23:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787242987; bh=nX4sx1FRpEMa84CGdbrmoCP6H7q2WWN23nS2L2nrCZA=; h=From:To:Cc:Subject:In-Reply-To:References:Date; b=Nxv859dkEHKPyVCse5t5aF4DPWyclCc04OXKHp3uViOGEFKqLG9kVJr2tPeAscjs3 GLfdFgSCPBrK0aGm4D4mh/vDavtfpYE1IiKivPnxF7ZVXaXt0gLokKj7N1nM6nbkcj FmL+jCXwKaeGbti6zDe09i6HOMIOXiXWVriqkcSACzS+54z9vjntOZeYgn4Yo5dkhv 7gUADHGBgIXKn+d6ETOn7RCSbIq6R7pvtRKO3DDvOacERm5p1XiOHaztsc3dZ/jiK8 FyEkNjun2icRaVxuPhXlhpFSra2ylqchTh7vaCjKH84S8xlFXbz1Zy4tZaHGb2kvY6 vJAVY9q5j9ZgA== From: Thomas Gleixner To: Usama Arif , Dmitry Ilvokhin Cc: peterz@infradead.org, andrealmeid@igalia.com, dave@stgolabs.net, dvhart@infradead.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, mingo@redhat.com, shuah@kernel.org, shakeel.butt@linux.dev, hannes@cmpxchg.org, riel@surriel.com, kernel-team@meta.com Subject: Re: [PATCH] futex: Avoid hash-bucket locking for mismatched waits In-Reply-To: <99a4d606-a53e-447e-951d-e356a97d16e9@linux.dev> References: <20260805132831.2852771-1-usama.arif@linux.dev> <87jyq2dji5.ffs@fw13> <87y0e8lmih.ffs@fw13> <87v79clmfu.ffs@fw13> <99a4d606-a53e-447e-951d-e356a97d16e9@linux.dev> Date: Thu, 20 Aug 2026 18:23:04 +0200 Message-ID: <87lda0iwwn.ffs@fw13> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain Usama! On Thu, Aug 20 2026 at 16:19, Usama Arif wrote: > On 14/08/2026 17:02, Thomas Gleixner wrote: >> --- a/kernel/futex/waitwake.c >> +++ b/kernel/futex/waitwake.c >> @@ -857,7 +857,21 @@ int futex_wait_setup(u32 __user *uaddr, >> CLASS(hbr, hbr)(&q->key); >> auto hb = hbr.hb; >> >> - futex_q_lock(q, hb); >> + futex_hb_waiters_inc(hb); >> + q->lock_ptr = &hb->lock; >> + >> + if (!spin_trylock(&hb->lock)) { >> + ret = get_user_inline(uval, uaddr); >> + if (ret) { >> + futex_hb_waiters_dec(hb); >> + return ret; >> + } >> + if (uval != val) { >> + futex_hb_waiters_dec(hb); >> + return -EWOULDBLOCK; >> + } >> + spin_lock(&hb->lock); >> + } >> >> ret = futex_get_value_locked(&uval, uaddr); >> > I like this approach; it makes the trade-off much better than my patch. > When spin_trylock() succeeds, the matching path performs only the existing > single user access, rather than the two accesses in my version. Correct. > Is it alright if I send v2 of the patch with your diff above? Or would you > like to send it? Go wild. It lacks an __acquire(q->lock_ptr) annotation and a comment for this trylock muck and the open coded hb_waiter_inc(). The latter can just refer to futex_q_lock(). Also the exit condition should be simplified into one "if (ret || uval != val)", which removes four lines of code. > Please let me know how you would like it attributed if I send it. Suggested-by is fine. Thanks, tglx