From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1946975Ab3BHUPj (ORCPT ); Fri, 8 Feb 2013 15:15:39 -0500 Received: from mail-oa0-f53.google.com ([209.85.219.53]:46600 "EHLO mail-oa0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932203Ab3BHUPh (ORCPT ); Fri, 8 Feb 2013 15:15:37 -0500 MIME-Version: 1.0 Reply-To: mtk.manpages@gmail.com In-Reply-To: <20130208191056.GA13674@redhat.com> References: <1358849741-9611-4-git-send-email-avagin@openvz.org> <1359486181-29088-1-git-send-email-avagin@openvz.org> <20130207173447.GA5888@redhat.com> <20130208191056.GA13674@redhat.com> From: "Michael Kerrisk (man-pages)" Date: Fri, 8 Feb 2013 21:15:16 +0100 Message-ID: Subject: Re: [PATCH 3/3] signalfd: add ability to read siginfo-s without dequeuing signals (v2) To: Oleg Nesterov Cc: Andrey Wagin , linux-kernel@vger.kernel.org, criu@openvz.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, Alexander Viro , "Paul E. McKenney" , David Howells , Dave Jones , Pavel Emelyanov , Cyrill Gorcunov Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 8, 2013 at 8:10 PM, Oleg Nesterov wrote: > On 02/08, Andrey Wagin wrote: >> >> 2013/2/7 Oleg Nesterov : >> > Andrey, sorry for delay. >> > >> > As for API, I leave this to you and Michael. Not that I like these >> > new flags, but I agree that pread() hack was not pretty too. >> > >> > On 01/29, Andrey Vagin wrote: >> >> +static ssize_t signalfd_peek(struct signalfd_ctx *ctx, >> >> + siginfo_t *info, loff_t *ppos, int queue_mask) >> >> +{ >> >> + loff_t seq = *ppos / sizeof(struct signalfd_siginfo); >> >> + int signr = 0; >> >> + >> >> + if (queue_mask & SIGQUEUE_PRIVATE) >> >> + signr = peek_signal(¤t->pending, >> >> + &ctx->sigmask, info, &seq); >> >> + else if (queue_mask & SIGQUEUE_SHARED) >> >> + signr = peek_signal(¤t->signal->shared_pending, >> >> + &ctx->sigmask, info, &seq); >> >> + (*ppos) += sizeof(struct signalfd_siginfo); >> > >> > Now that this can work even with normal read(), we will actually change >> > f_pos. Then perhaps signalfd_fops->llseek() should work too. But this >> > is minor... >> >> lseek works only if FMODE_LSEEK is set. >> >> You have explained why read&lseek have strange semantics for SIGNALFD_PEEK. >> >> >Damn. But after I wrote this email I realized that llseek() probably can't >> > work. Because peek_offset/f_pos/whatever has to be shared with all processes >> > which have this file opened. > > Yes. but I thought you decided to ignore this oddity ;) > >> So I want to suggest a way how to forbid read() for SIGNALFD_PEEK. >> file->f_pos can be initialized to -1. read() returns EINVAL in this >> case. In a man page we will write that signals can be dumped only with >> help pread(). Is it overload or too ugly? > > Well. I do not know. Up to you and Michael. > > But honestly, I can't say this all looks really nice. And why do we > need SIGNALFD_PEEK then? It surely is no beauty. The hope is at least to make it less ugly than it was. > Seriously, perhaps we should simply add signalfd_fops->ioctl() for PEEK. > Or add PTRACE_{PEEK,POKE}_SIGNAL which looks even logical and useful... > And much simpler/straightforward. > > But I am not going to argue. I suppose I had wondered along similar lines, but in a slightly different direction: would the use of a /proc interface to get the queued signals make some sense? Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Author of "The Linux Programming Interface"; http://man7.org/tlpi/