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 X-Spam-Level: X-Spam-Status: No, score=-0.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 628FDC432C3 for ; Thu, 14 Nov 2019 13:22:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3E27620723 for ; Thu, 14 Nov 2019 13:22:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726327AbfKNNWN convert rfc822-to-8bit (ORCPT ); Thu, 14 Nov 2019 08:22:13 -0500 Received: from eu-smtp-delivery-151.mimecast.com ([207.82.80.151]:28644 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726190AbfKNNWM (ORCPT ); Thu, 14 Nov 2019 08:22:12 -0500 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-172-PjHJCdYGMqyCEpwqMfyeLA-1; Thu, 14 Nov 2019 13:22:04 +0000 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Thu, 14 Nov 2019 13:22:03 +0000 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Thu, 14 Nov 2019 13:22:03 +0000 From: David Laight To: 'Peter Zijlstra' CC: Thomas Gleixner , LKML , "x86@kernel.org" , Linus Torvalds , Andy Lutomirski , "Stephen Hemminger" , Willy Tarreau , "Juergen Gross" , Sean Christopherson , "H. Peter Anvin" Subject: RE: [patch V2 11/16] x86/ioperm: Share I/O bitmap if identical Thread-Topic: [patch V2 11/16] x86/ioperm: Share I/O bitmap if identical Thread-Index: AQHVmTnGMlCgX/wyC0irSyeDShf4MaeKgnHAgAAklgCAAAMfoA== Date: Thu, 14 Nov 2019 13:22:03 +0000 Message-ID: <2e02321be78d43e9b7d7ead29a10b1d9@AcuMS.aculab.com> References: <20191111220314.519933535@linutronix.de> <20191111223052.603030685@linutronix.de> <20191112091521.GX4131@hirez.programming.kicks-ass.net> <20191114130926.GP4114@hirez.programming.kicks-ass.net> In-Reply-To: <20191114130926.GP4114@hirez.programming.kicks-ass.net> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 X-MC-Unique: PjHJCdYGMqyCEpwqMfyeLA-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Peter Zijlstra > On Thu, Nov 14, 2019 at 11:02:01AM +0000, David Laight wrote: > > From: Peter Zijlstra > > > Sent: 12 November 2019 09:15 > > ... > > > > + /* > > > > + * If the bitmap is not shared, then nothing can take a refcount as > > > > + * current can obviously not fork at the same time. If it's shared > > > > + * duplicate it and drop the refcount on the original one. > > > > + */ > > > > + if (refcount_read(&iobm->refcnt) > 1) { > > > > + iobm = kmemdup(iobm, sizeof(*iobm), GFP_KERNEL); > > > > + if (!iobm) > > > > + return -ENOMEM; > > > > + io_bitmap_exit(); > > > refcount_set(&iobm->refcnd, 1); > > > > } > > > > What happens if two threads of the same process enter the above > > at the same time? > > Suppose there's just the two threads, and both will change it. Then both > do copy-on-write and the original gets freed. I was probably forgetting that the linux kernel uses (more or less) full 'process' structures for threads, rather than separate 'thread' data areas. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)