Re: [PATCH 0/3] signalfd: a kernel interface for dumping pending signals

On Wed, Jan 23, 2013 at 01:11:42PM +0100, Michael Kerrisk (man-pages) wrote:

Hi Andrey,

On Wed, Jan 23, 2013 at 12:03 PM, Andrew Vagin [off-list ref] wrote:

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On Wed, Jan 23, 2013 at 05:19:24AM +0100, Michael Kerrisk (man-pages) wrote:

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Hi Andrey,

On Tue, Jan 22, 2013 at 11:15 AM, Andrey Vagin [off-list ref] wrote:

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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

quoted

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.

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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;

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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);

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    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.)

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* 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.

offset is usual in bytes.

Lets imagine that we have a file, which contains siginfo-s.
If "pos" is offset in bytes, the same code can reads siginfo-s from the
file and from signalfd.

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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

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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.

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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())?

Only for the first question.

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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?

It's true, if both SFD_QUEUE flags was specified.
If only one for these flags is specified, the 'offset' argument is a
sequnce number in a proper list.

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?

No. crtools reads signals from each queue separately.

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?

Looks reasonable.

Thanks,

Michael