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

[Michael Kerrisk (man-pages)]

Hi Andrey,

On Wed, Jan 23, 2013 at 12:03 PM, Andrew Vagin <avagin at 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 at 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