On 11/14/19 8:27 AM, Jens Axboe wrote:
> On 11/14/19 8:20 AM, Jens Axboe wrote:
>> On 11/14/19 8:19 AM, Rasmus Villemoes wrote:
>>> On 14/11/2019 16.09, Jens Axboe wrote:
>>>> On 11/14/19 7:12 AM, Rasmus Villemoes wrote:
>>>
>>>>> So, I can't really think of anybody that might be relying on inheriting
>>>>> a signalfd instead of just setting it up in the child, but changing the
>>>>> semantics of it now seems rather dangerous. Also, I _can_ imagine
>>>>> threads in a process sharing a signalfd (initial thread sets it up and
>>>>> blocks the signals, all threads subsequently use that same fd), and for
>>>>> that case it would be wrong for one thread to dequeue signals directed
>>>>> at the initial thread. Plus the lifetime problems.
>>>>
>>>> What if we just made it specific SFD_CLOEXEC?
>>>
>>> O_CLOEXEC can be set and removed afterwards. Sure, we're far into
>>> "nobody does that" land, but having signalfd() have wildly different
>>> semantics based on whether it was initially created with O_CLOEXEC seems
>>> rather dubious.
>>>
>>> I don't want to break
>>>> existing applications, even if the use case is nonsensical, but it is
>>>> important to allow signalfd to be properly used with use cases that are
>>>> already in the kernel (aio with IOCB_CMD_POLL, io_uring with
>>>> IORING_OP_POLL_ADD). Alternatively, if need be, we could add a specific
>>>> SFD_ flag for this.
>>>
>>> Yeah, if you want another signalfd flavour, adding it via a new SFD_
>>> flag seems the way to go. Though I can't imagine the resulting code
>>> would be very pretty.
>>
>> Well, it's currently _broken_ for the listed in-kernel use cases, so
>> I think making it work is the first priority here.
>
> How about something like this, then? Not tested.
Tested, works for me. Here's the test case I used. We setup a signalfd
with SIGALRM, and arm a timer for 100msec. Then we queue a poll for the
signalfd, and wait for that to complete with a timeout of 1 second. If
we time out waiting for the completion, we failed. If we do get a
completion but we don't have POLLIN set, we failed.
#include <unistd.h>
#include <sys/signalfd.h>
#include <sys/poll.h>
#include <sys/time.h>
#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <liburing.h>
#define SFD_TASK 00000001
int main(int argc, char *argv[])
{
struct __kernel_timespec ts;
struct io_uring_sqe *sqe;
struct io_uring_cqe *cqe;
struct io_uring ring;
struct itimerval itv;
sigset_t mask;
int sfd, ret;
sigemptyset(&mask);
sigaddset(&mask, SIGALRM);
sigprocmask(SIG_BLOCK, &mask, NULL);
sfd = signalfd(-1, &mask, SFD_NONBLOCK | SFD_CLOEXEC | SFD_TASK);
if (sfd < 0) {
if (errno == EINVAL) {
printf("Not supported\n");
return 0;
}
perror("signalfd");
return 1;
}
memset(&itv, 0, sizeof(itv));
itv.it_value.tv_sec = 0;
itv.it_value.tv_usec = 100000;
setitimer(ITIMER_REAL, &itv, NULL);
io_uring_queue_init(32, &ring, 0);
sqe = io_uring_get_sqe(&ring);
io_uring_prep_poll_add(sqe, sfd, POLLIN);
io_uring_submit(&ring);
ts.tv_sec = 1;
ts.tv_nsec = 0;
ret = io_uring_wait_cqe_timeout(&ring, &cqe, &ts);
if (ret < 0) {
fprintf(stderr, "Timed out waiting for cqe\n");
ret = 1;
} else {
if (cqe->res < 0) {
fprintf(stderr, "cqe failed with %d\n", cqe->res);
ret = 1;
} else if (!(cqe->res & POLLIN)) {
fprintf(stderr, "POLLIN not set in result mask?\n");
ret = 1;
} else {
ret = 0;
}
}
io_uring_cqe_seen(&ring, cqe);
io_uring_queue_exit(&ring);
close(sfd);
return ret;
}
--
Jens Axboe
