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

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 [off-list ref] wrote:

quoted

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

quoted

Cc: Serge Hallyn <redacted>
Cc: Oleg Nesterov <redacted>
Cc: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
Cc: "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
Cc: Al Viro <viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
Cc: Pavel Emelyanov <redacted>
CC: Cyrill Gorcunov <redacted>
Cc: Michael Kerrisk <redacted>

--
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/