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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 18 Dec 2019 16:51:27 -0800
Message-ID: <CAHk-=wgMiTbRPp6Fx_A4YV+9xL7dc2j0Dj3NTFDPRfjsjLQTWw@mail.gmail.com>
Subject: Re: [PATCH 0/2] pipe: Fixes [ver #2]
To:     Josh Triplett <josh@joshtriplett.org>
Cc:     Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        Akemi Yagi <toracat@elrepo.org>, DJ Delorie <dj@redhat.com>,
        David Sterba <dsterba@suse.cz>,
        David Howells <dhowells@redhat.com>,
        Eric Biggers <ebiggers@kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Vincent Guittot <vincent.guittot@linaro.org>
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On Wed, Dec 18, 2019 at 4:14 PM Josh Triplett <josh@joshtriplett.org> wrote:
>
> Er, wrong file. That's the original patch; the attached patch is the
> right one.

This looks correct to me.

If I were to actually commit it, the "split into two waitqueues" would
be a separate patch from the "use wait_event_interruptible_exclusive()
and add "wake_next_reader/writer logic", but for testing purposes the
unified patch was simpler, and your forward port looks good to me.

I ran the original patch for a couple of days, and didn't see any
other issues than the 'make' thing in F30. It was all good with my
self-build make.

But that "ran for a couple of days" wasn't all that stress-full. I did
do the "verify that the thundering herd is gone" test - including that
silly test-case here again:

    #include <unistd.h>

    int main(int argc, char **argv)
    {
        int fd[2], counters[2];

        pipe(fd);
        counters[0] = 0;
        counters[1] = -1;
        write(fd[1], counters, sizeof(counters));

        /* 64 processes */
        fork(); fork(); fork(); fork(); fork(); fork();

        do {
                int i;
                read(fd[0], &i, sizeof(i));
                if (i < 0)
                        continue;
                counters[0] = i+1;
                write(fd[1], counters, (1+(i & 1)) *sizeof(int));
        } while (counters[0] < 1000000);
        return 0;
    }

where you can tweak the numbers - add another fork() or two to create
even more pipe waiters, and maybe change the final count exit value to
match whatever hw performance you have.

         Linus