From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755123Ab3AWLE1 (ORCPT ); Wed, 23 Jan 2013 06:04:27 -0500 Received: from relay.parallels.com ([195.214.232.42]:53657 "EHLO relay.parallels.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754597Ab3AWLEZ (ORCPT ); Wed, 23 Jan 2013 06:04:25 -0500 Date: Wed, 23 Jan 2013 15:03:23 +0400 From: Andrew Vagin To: "Michael Kerrisk (man-pages)" CC: Andrey Vagin , , , , , Serge Hallyn , Oleg Nesterov , Andrew Morton , "Eric W. Biederman" , Al Viro , Pavel Emelyanov , Cyrill Gorcunov Subject: Re: [PATCH 0/3] signalfd: a kernel interface for dumping pending signals Message-ID: <20130123110323.GA23139@paralelels.com> References: <1358849741-9611-1-git-send-email-avagin@openvz.org> MIME-Version: 1.0 Content-Type: text/plain; charset="koi8-r" Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-Originating-IP: [10.30.16.48] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 23, 2013 at 05:19:24AM +0100, Michael Kerrisk (man-pages) wrote: > Hi Andrey, > > On Tue, Jan 22, 2013 at 11:15 AM, Andrey Vagin wrote: > > This patch set adds ability to choose a signal queue and > > to read signals without dequeuing them. > > > > Three new flags are added: > > SFD_SHARED_QUEUE -- reads will be from process-wide shared signal queue > > SFD_PER_THREAD_QUEUE -- reads will be from per-thread signal queue > > SFD_PEEK -- don't dequeue signals > > A fuller description of the patch, including information that was in > previous versions of this patch would be helpful. Let me see if I can > summarize/fill out the API side of things, and ask a few questions > along the way (yes, I could answer some of the questions by checking > the code, but I want to know what the *intended* behavior is). > > The patch series adds a total of 4 flags to signalfd(). In addition to > those you list above, the other is In additional we can say, that this patch series adds three orthogonal, independent groups of flags. * SFD_RAW * SFD_PEEK * SFD_SHARED_QUEUE, SFD_PER_THREAD_QUEUE > > SFD_RAW -- return raw siginfo structs when reading, rather than signalfd_siginfo > > The intention is that these flags be used in conjunction with pread(), > to peek at queued signals. The 'offset' argument is treated as a > position. Thus, for example, to non-destructively read all of the > per-thread signals in raw form from the per-thread queue, one would > write > siginfo_t *buf; > fd = signalfd(-1, SFD_PER_THREAD_QUEUE | SFD_RAW | SFD_PEEK) > for (j = 0; ; j++) { > s = pread(fd, buf, ocunt, j) s = pread(fd, buf + j, sizeof(siginfo_t), j); > if (s <= 0) /* No more signals */ > break; > } This examples reads signals one by one or siginfo_t *buf = NULL; unsigned long buf_size = 0, nr = 0; int ret; while (1) { bug_size += PAGE_SIZ; buf = realloc(buf, buf_size); if (buf == NULL) goto err; ret = pread(fd, buf + nr, sizeof(siginfo_t), nr); if (ret == -1) goto err; nr += ret / sizeof(siginfo_t); if (ret < PAGE_SIZE) /* No more signals */ break; } pread() can read more than one signal. * The interface of signalfd could be a bit more predictable, if we will treat pos as offset in bytes, not in elements. pread(fd, buf, sizeof(siginfo_t), i * sizeof(siginfo_t)) - reads a signal with a sequence number i in a queue. > > Right? > > Now some questions. I don't require all of the following, but I'm > wanting to know what's possible, for documentation purposes. > > Q1: with this patch series, is it permissible to specify > SFD_PER_THREAD_QUEUE or SFD_SHARED_QUEUE without specifying either > SFD_PEEK or SFD_QUEUE? In other words, can one do traditional > signalfd_siginfo reads, but selecting from a specific queue. Yes, we can > > Q2: Is it possible to specify SFD_PEEK without SFD_RAW, so that one > can peek at siginfo structs rather than signalfd_siginfo structs? Yes, it is possible. read() and pread() returns signalfd_siginfo structs in this case. > > Q3: Is it possible to specify SFD_RAW without SFD_PEEK, so that one > can destructively read signalfd_siginfo structs? Can that be done > using any read interface (read(), pread(), etc.)? Yes, it is possible too. read() will return siginfo structs. > > Q4: Is it possible to specify both SFD_PER_THREAD_QUEUE and > SFD_SHARED_QUEUE? In that case, in what order are signals read from > the two queues? > It is equal to the case, when none of these flags are not specified. And it is equal to what we had before this patches. signalfd() reads signals from a private queue, then from a shared queue. ps: A good analogy: signalfd looks like a pipe if SFD_PEEK are not specified, otherwise it looks like a file > Thanks, > > Michael > > > Cc: Serge Hallyn > > Cc: Oleg Nesterov > > Cc: Andrew Morton > > Cc: "Eric W. Biederman" > > Cc: Al Viro > > Cc: Pavel Emelyanov > > CC: Cyrill Gorcunov > > Cc: Michael Kerrisk > > > > -- > > 1.7.11.7 > > > > > > -- > Michael Kerrisk > Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ > Author of "The Linux Programming Interface"; http://man7.org/tlpi/