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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 11 May 2020 12:33:58 -0700
Message-ID: <CAHk-=wi=SbaC20nRx5tmAZ2_tOpVOq7469V+KvZU9=4yvfvmnA@mail.gmail.com>
Subject: Re: I disabled more compiler warnings..
To:     Paul Smith <psmith@gnu.org>
Cc:     David Laight <David.Laight@aculab.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Mon, May 11, 2020 at 10:59 AM Paul Smith <psmith@gnu.org> wrote:
>
> As with all single-threaded applications, though, the problem is the
> difficulty (in a portable way) of handling both signals and wait*(2)
> reliably...

I do wonder if GNU make isn't making it worse for itself by blocking SIGCHLD.

I wonder if you could just have three different file descriptors:

 - the "current token file descriptor"

 - a /dev/null file descriptor

 - the jobserver pipe file descriptor. This is left blocking.

and then make the rule be that

 - the SIGCHLD handler just does

        dup2(nullfd, jobserverfd);

   This guarantees that any read() that was interrupted by a SIGCHLD
will now reliably return immediately. But it also guarantees that if
we raced, and the SIGCHLD happened just *before* we did the read(),
the read() will also exit immediately.

 - the waiting code does

        check_child_status();
        for (;;) {
            ret = read(jobserverfd, buffer, 1);
            if (ret == 1) {
                .. we got the token successfully, do whatever ..
                return;
            }
            /* The read might fail because of SIGCHLD - either we got
interrupted, or the fd was replaced with /dev/null */
            /* First, re-instate the pipe binding */
            dup2(pipefd, jobserverfd);
            /* Then do the child status stuff again */
            check_child_status();
            /* Ok, we can restart, there's no races with SIGCHLD */
         }

which fundamentally doesn't have any races. Look, ma, no need for
nonblocking reads, or pselect().

I don't know. Maybe I missed something.

               Linus