Date: Wed, 2 Jan 2019 13:53:00 +0100 From: Willem Jan Withagen <wjw@digiware.nl> To: Alan Somers <asomers@freebsd.org> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: Using kqueue with aio_read/write Message-ID: <016fda76-8f02-ca62-af7d-062262bd4a09@digiware.nl> In-Reply-To: <CAOtMX2ics595SQTDR=ORF8YCTOgm9JSk-Nv1T89O6iCOp5O4fg@mail.gmail.com> References: <8753521a-4555-ec2a-5efc-dee2660b4d9b@digiware.nl> <CAOtMX2iOy4Uf%2B9%2BuYhbX-wXJ68E57CjTW0aLPsa3dH__n4oP_w@mail.gmail.com> <969d9a38-d3dd-78d0-c974-ba14ec4747db@digiware.nl> <CAOtMX2ics595SQTDR=ORF8YCTOgm9JSk-Nv1T89O6iCOp5O4fg@mail.gmail.com>
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On 02/01/2019 03:23, Alan Somers wrote:
> On Tue, Jan 1, 2019 at 6:56 PM Willem Jan Withagen <wjw@digiware.nl> wrote:
>> On 28/12/2018 02:47, Alan Somers wrote:
>>> On Thu, Dec 27, 2018 at 6:15 PM Willem Jan Withagen <wjw@digiware.nl> wrote:
>>>> Hi,
>>>>
>>>> Im trying to understand why I cannot get so code to work.
>>>> This is the smallest extract I can make to show my problem.
>>>>
>>>> I would expect the kevent() call to return every timeo tick.
>>>> Even if I tell it NOT to time-out I get these spurts of errors
>>>>
>>>> Since there is nothing to trigger the AIO-event, I would expect kqueue
>>>> to hold indefinitly.
>>>>
>>>> But it does not generate anything other than errors
>>>> And instead it repeatedly complains that there is a permission error:
>>>> get_events_kevent: EV_Error(1) kevent(): Operation not permitted
>>>>
>>>> But I'm not getting where that would the case...
>>>>
>>>> Surely a pilot error, but I do overlook it al the time.
>>>> So suggestions are welcome.
>>>>
>>>> Thanx,
>>>> --WjW
>>>>
>>>> #include <aio.h>
>>>> #include <errno.h>
>>>> #include <fcntl.h>
>>>> #include <stdio.h>
>>>> #include <stdlib.h>
>>>> #include <string.h>
>>>> #include <sys/stat.h>
>>>> #include <sys/event.h>
>>>> #include <unistd.h>
>>>>
>>>> #define BUFFER_SIZE 512
>>>> #define MAX_EVENTS 32
>>>>
>>>> #define FILENAME "/tmp/aio_test"
>>>> char filename[256];
>>>> int fd;
>>>> int done = 0;
>>>>
>>>> void get_events_kevent(int fd, int kq)
>>>> {
>>>> printf("get_events function fd = %d, kq = %d\n", fd, kq);
>>>> int i = 0, errcnt = 0, err, ret, reterr, rev;
>>>> int search = 1;
>>>>
>>>> int timeout_ms = 10;
>>>> struct timespec timeo = {
>>>> timeout_ms / 1000,
>>>> (timeout_ms % 1000) * 1000 * 1000
>>>> };
>>>> struct kevent filter[16];
>>>> struct kevent changed[16];
>>>>
>>>> EV_SET(&filter[0], fd, EVFILT_AIO,
>>>> EV_ADD,
>>>> 0, 0, 0 );
>>> This is the first problem. There's no need to explicitly set
>>> EVFILT_AIO on the kqueue. It gets set by the aio_read(2) or similar
>>> syscall. And this invocation wouldn't be correct anyway, because for
>>> AIO the ident field refers to the address of the struct aiocb, not the
>>> file descriptor. If the only events you care about are AIO, then you
>>> can pass NULL as the filter argument to kevent. I suspect this is the
>>> cause of your problem. The kernel probably thinks you're trying to
>>> register for an aiocb that's outside of your address space or
>>> something like that.
>>>
>>>
>>>> while (!done) {
>>>> printf("+");
>>>> rev = kevent(kq, filter, 1, changed, 16, 0); //&timeo);
>>>> if (rev < 0) {
>>>> perror("kevent error");
>>>> } else if (rev == 0) {
>>>> printf("T");
>>>> } else {
>>>> printf("rev(%d)\n", rev);
>>>> if (changed[0].flags == EV_ERROR) {
>>>> errno = changed[0].data;
>>>> printf( "%s: EV_Error(%d) kevent(): %s\n", __func__, errno,
>>>> strerror(errno));
>>>> memset(&changed[0], 0, sizeof(struct kevent));
>>>> } else {
>>>> err = aio_error((struct aiocb*)changed[0].udata);
>>> No need to call aio_error(2) after kevent(2) returns. You can go
>>> straight to aio_return. aio_error shouldn't hurt, but it isn't
>>> necessary.
>> According to kevent(2) calling kevent can return errors on the called
>> aio_calls.
>> It then returns with EV_ERROR in flags, and errno is stored in the
>> event.data.
>>
>> But what would be going on when the event's flag contains EV_ERROR but
>> event's data is still 0???
>>
>> the udata field still seems to point to the aio data that was passed
>> into the aio block when calling aio_read().
>>
>> Should I ignore this as a non-error?
>>
>> --WjW
> Are you sure you bzero()ed your aiocb before initializing it? Any
> stack garbage that was present in its
> aio_sigevent.sigev_notify_kevent_flags field will be dutifully copied
> into the returned kevent. And in any case, the definitive way to get
> the final status of a completed aio operation is with aio_return.
That seems to help in getting things clear...
-13> 2019-01-02 13:32:31.834 dc15a80 1 bdev:327 paio
get_next_completed processing event i = 0 aio_return(22) (22) Invalid
argument
Disadvantage is that it is not clear yet which of the many arguments
that is?
--WjW
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