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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 81DE0C43334 for ; Fri, 17 Jun 2022 14:43:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1382848AbiFQOni (ORCPT ); Fri, 17 Jun 2022 10:43:38 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47164 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1382845AbiFQOnZ (ORCPT ); Fri, 17 Jun 2022 10:43:25 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 25B9B338BD for ; Fri, 17 Jun 2022 07:43:24 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1655477003; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ELNIoDd4jGFrzzoRGlYbdPQ/VgoPmPjRtlTV2XWJ2HY=; b=HxQfDw+oiPmrR3pm+VDDO0iWBdBTuUUnIHliRwocvF+2LoZxm31SXqI5pkqpkzZ6Hc6iQL NyWzdJjVAf2YjpRG7xidW/PJJVWdGn+Mjk4fDX9cs25SN2agT1oY81Cj/hSWz1b7j9MUt7 a+hcuDuHxJlBMP32fOOFNasTCh9p0s0= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-226-yzMJJcfxPy29hkN2pHmvPw-1; Fri, 17 Jun 2022 10:43:18 -0400 X-MC-Unique: yzMJJcfxPy29hkN2pHmvPw-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 8B89E101AA47; Fri, 17 Jun 2022 14:43:17 +0000 (UTC) Received: from [10.22.18.98] (unknown [10.22.18.98]) by smtp.corp.redhat.com (Postfix) with ESMTP id 162B7815B; Fri, 17 Jun 2022 14:43:17 +0000 (UTC) Message-ID: <2dd754f9-3a79-ed17-e423-6b411c3afb69@redhat.com> Date: Fri, 17 Jun 2022 10:43:16 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.1 Subject: Re: [PATCH] locking/rwlocks: do not starve writers Content-Language: en-US To: Peter Zijlstra , Eric Dumazet Cc: Linus Torvalds , linux-kernel , Eric Dumazet , Ingo Molnar , Boqun Feng , Will Deacon , Roman Penyaev , Shakeel Butt References: <20220617091039.2257083-1-eric.dumazet@gmail.com> From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.11.54.5 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/17/22 08:07, Peter Zijlstra wrote: > On Fri, Jun 17, 2022 at 02:10:39AM -0700, Eric Dumazet wrote: >> --- a/kernel/locking/qrwlock.c >> +++ b/kernel/locking/qrwlock.c >> @@ -23,16 +23,6 @@ void queued_read_lock_slowpath(struct qrwlock *lock) >> /* >> * Readers come here when they cannot get the lock without waiting >> */ >> - if (unlikely(in_interrupt())) { >> - /* >> - * Readers in interrupt context will get the lock immediately >> - * if the writer is just waiting (not holding the lock yet), >> - * so spin with ACQUIRE semantics until the lock is available >> - * without waiting in the queue. >> - */ >> - atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED)); >> - return; >> - } >> atomic_sub(_QR_BIAS, &lock->cnts); >> >> trace_contention_begin(lock, LCB_F_SPIN | LCB_F_READ); > This is known to break tasklist_lock. > We certainly can't break the current usage of tasklist_lock. I am aware of this problem with networking code and is thinking about either relaxing the check to exclude softirq or provide a read_lock_unfair() variant for networking use. I think tasklist_lock isn't taken from softirq context, but I may be wrong. Providing a read_lock_unfair() will require quite a bit of work in the supporting infrastructure as well. Cheers, Longman