From: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
To: Andrew Vagin <avagin@parallels.com>
Cc: Andrey Vagin <avagin@openvz.org>,
linux-kernel@vger.kernel.org, criu@openvz.org,
linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org,
Serge Hallyn <serge.hallyn@canonical.com>,
Oleg Nesterov <oleg@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
"Eric W. Biederman" <ebiederm@xmission.com>,
Al Viro <viro@zeniv.linux.org.uk>,
Pavel Emelyanov <xemul@parallels.com>,
Cyrill Gorcunov <gorcunov@openvz.org>
Subject: Re: [PATCH 0/3] signalfd: a kernel interface for dumping pending signals
Date: Wed, 23 Jan 2013 13:11:42 +0100 [thread overview]
Message-ID: <CAKgNAkjCX4L6R3X7uaLufjY61ZszQTWbD+bLqadGRB-HLNKOrw@mail.gmail.com> (raw)
In-Reply-To: <20130123110323.GA23139@paralelels.com>
Hi Andrey,
On Wed, Jan 23, 2013 at 12:03 PM, Andrew Vagin <avagin@parallels.com> wrote:
> 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 <avagin@openvz.org> 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
Thanks. Nice summary.
>> 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.
(Thanks for the reminder on that last point.)
> * 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.
Can you explain what you mean by "more predictable"? It's not clear to me.
>> 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.
3 * yes is nice!
For which of the above 3 questions was the answer "No" with the
previous version of these patches (the version that specified queue
selection in pread())?
>> 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.
So, the 'offset' argument of pread() is interpreted by considering the
per-thread and shared queue as one concatenated list, right?
If yes to the previous question, then from an API design point of view
that seems odd: it exposes an implementation detail. Is specifying
both SFD_PER_THREAD_QUEUE and SFD_SHARED_QUEUE usefule for
checkpoint/restore? I almost wonder if, when SFD_PEEK is specified, a
requirement should be enforced that SFD_PER_THREAD_QUEUE or
SFD_SHARED_QUEUE, but not both, must be specified. What do you think?
Thanks,
Michael
next prev parent reply other threads:[~2013-01-23 12:12 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-01-22 10:15 [PATCH 0/3] signalfd: a kernel interface for dumping pending signals Andrey Vagin
2013-01-22 10:15 ` [PATCH 1/3] signal: add a helper for dequeuing signals from a specified queue Andrey Vagin
2013-02-05 12:00 ` [PATCH 1/3] signal: add a helper for dequeuing signals from a specified queue (v2) Andrey Vagin
2013-01-22 10:15 ` [PATCH 2/3] signalfd: add ability to choose a private or shared queue Andrey Vagin
2013-02-07 18:17 ` Oleg Nesterov
2013-02-08 0:35 ` Michael Kerrisk (man-pages)
2013-02-08 19:12 ` Oleg Nesterov
2013-01-22 10:15 ` [PATCH 3/3] signalfd: add ability to read siginfo-s without dequeuing signals Andrey Vagin
2013-01-29 19:03 ` [PATCH 3/3] signalfd: add ability to read siginfo-s without dequeuing signals (v2) Andrey Vagin
[not found] ` <CAKgNAkgQA=zK=2ZnytPFU=DH6jr0sja0iy6K+j6c7unheLFniQ@mail.gmail.com>
2013-02-02 7:15 ` Andrey Wagin
2013-02-07 17:34 ` Oleg Nesterov
2013-02-07 21:13 ` Andrey Wagin
2013-02-08 0:51 ` Michael Kerrisk (man-pages)
2013-02-08 19:10 ` Oleg Nesterov
2013-02-08 20:15 ` Michael Kerrisk (man-pages)
2013-02-09 18:22 ` Oleg Nesterov
2013-02-09 22:53 ` Michael Kerrisk (man-pages)
2013-02-10 10:04 ` [CRIU] " Andrew Vagin
2013-02-11 16:47 ` Oleg Nesterov
2013-02-10 10:07 ` Andrew Vagin
2013-02-11 9:29 ` Denys Vlasenko
2013-02-11 10:59 ` [CRIU] " Andrew Vagin
2013-02-11 14:46 ` Denys Vlasenko
2013-02-11 14:53 ` Pavel Emelyanov
2013-02-11 17:25 ` Denys Vlasenko
2013-02-12 14:50 ` Pavel Emelyanov
2013-01-23 4:19 ` [PATCH 0/3] signalfd: a kernel interface for dumping pending signals Michael Kerrisk (man-pages)
2013-01-23 11:03 ` Andrew Vagin
2013-01-23 12:11 ` Michael Kerrisk (man-pages) [this message]
2013-01-23 13:03 ` Andrew Vagin
2013-02-07 18:20 ` Oleg Nesterov
2013-02-08 0:36 ` Michael Kerrisk (man-pages)
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