linux-kernel - Re: splice(-> FIFO) never wakes up inotify IN

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Date:   Mon, 26 Jun 2023 17:00:01 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ahelenia Ziemiańska 
        <nabijaczleweli@...ijaczleweli.xyz>
Cc:     Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: splice(-> FIFO) never wakes up inotify IN_MODIFY?

On Mon 26-06-23 16:25:41, Ahelenia Ziemiańska wrote:
> On Mon, Jun 26, 2023 at 03:51:59PM +0200, Jan Kara wrote:
> > On Mon 26-06-23 14:57:55, Ahelenia Ziemiańska wrote:
> > > On Mon, Jun 26, 2023 at 02:19:42PM +0200, Ahelenia Ziemiańska wrote:
> > > > > splice(2) differentiates three different cases:
> > > > >         if (ipipe && opipe) {
> > > > > ...
> > > > >         if (ipipe) {
> > > > > ...
> > > > >         if (opipe) {
> > > > > ...
> > > > > 
> > > > > IN_ACCESS will only be generated for non-pipe input
> > > > > IN_MODIFY will only be generated for non-pipe output
> > > > >
> > > > > Similarly FAN_ACCESS_PERM fanotify permission events
> > > > > will only be generated for non-pipe input.
> > > Sorry, I must've misunderstood this as "splicing to a pipe generates
> > > *ACCESS". Testing reveals this is not the case. So is it really true
> > > that the only way to poll a pipe is a sleep()/read(O_NONBLOCK) loop?
> > So why doesn't poll(3) work? AFAIK it should...
> poll returns instantly with revents=POLLHUP for pipes that were closed
> by the last writer.
> 
> Thus, you're either in a hot loop or you have to explicitly detect this
> and fall back to sleeping, which defeats the point of polling:

I see. There are two ways around this:

a) open the file descriptor with O_RDWR (so there's always at least one
writer).

b) when you get POLLHUP, just close the fd and open it again.

In these cases poll(3) will behave as you need (tested)...

								Honza

> -- >8 --
> #define _GNU_SOURCE
> #include <errno.h>
> #include <fcntl.h>
> #include <poll.h>
> #include <stdio.h>
> #include <unistd.h>
> int main() {
>   char buf[64 * 1024];
>   struct pollfd pf = {.fd = 0, .events = POLLIN};
>   size_t consec = 0;
>   for (ssize_t rd;;) {
>     while (poll(&pf, 1, -1) <= 0)
>       ;
>     if (pf.revents & POLLIN) {
>       while ((rd = read(0, buf, sizeof(buf))) == -1 && errno == EINTR)
>         ;
>       fprintf(stderr, "\nrd=%zd: %m\n", rd);
>     }
>     if (pf.revents & POLLHUP) {
>       if (!consec++)
>         fprintf(stderr, "\n\tPOLLHUPs");
>       fprintf(stderr, "\r%zu", consec);
>     } else
>       consec = 0;
>   }
> }
> -- >8 --
> 
> And
> -- >8 --
> $ ./rdr < fifo
> 
> rd=12: Success
> 
> 1779532 POLLHUPs
> rd=5: Success
> 
> 945087  POLLHUPs
> rd=12: Success
> ^C
> -- >8 --
> corresponding to
> -- >8 --
> $ cat > fifo
> abc
> def
> ghi
> ^D
> $ echo zupa > fifo
> $ cat > fifo
> as
> dsaa
> asd
> ^C
> -- >8 --


-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR