From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759413Ab3BGVS5 (ORCPT ); Thu, 7 Feb 2013 16:18:57 -0500 Received: from mail-vc0-f181.google.com ([209.85.220.181]:34981 "EHLO mail-vc0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759281Ab3BGVSz (ORCPT ); Thu, 7 Feb 2013 16:18:55 -0500 MIME-Version: 1.0 In-Reply-To: <20130207173447.GA5888@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> Date: Fri, 8 Feb 2013 01:13:13 +0400 Message-ID: Subject: Re: [PATCH 3/3] signalfd: add ability to read siginfo-s without dequeuing signals (v2) From: Andrey Wagin To: Oleg Nesterov Cc: 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 , Michael Kerrisk , 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 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. > > Suppose that the task forks after sys_signalfd(). Now if parent or child > do llseek this affects them both. This is insane because signalfd is > "strange" to say at least, fork/dup/etc inherits signalfd_ctx but not the > "source" of the data. 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? > > Hmm. but since it works with read(), we shouldn't increment *ppos unless > signalfd_copyinfo() succeeds? No, we shouldn't. > > Btw, why do you pass seq by reference? Looks unneeded. You are right. I created this code for reading signals from both queues, but then we decided to forbid using SIGNALFD_PEEK for both queues simultaneously. Oleg, thank you for the comments. I'm waiting an answer on the question and after that I'm going to send a final version. > > Oleg. >