linux-kernel - Re: [GIT PULL] pipe: Notification queue preparation

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Message-ID: <CAHk-=whw+R5GVQdpV6J_5afQ=76vtBPzBPRj6-zG1tnhT32Pag@mail.gmail.com>
Date:   Thu, 5 Dec 2019 09:33:04 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Sterba <dsterba@...e.cz>,
        David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>, raven@...maw.net,
        Christian Brauner <christian@...uner.io>,
        keyrings@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-block <linux-block@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] pipe: Notification queue preparation

On Thu, Dec 5, 2019 at 9:22 AM David Sterba <dsterba@...e.cz> wrote:
>
> I rerun the test again (with a different address where it's stuck), there's
> nothing better I can get from the debug info, it always points to pipe_wait,
> disassembly points to.

Hah. I see another bug.

"pipe_wait()" depends on the fact that all events that wake it up
happen with the pipe lock held.

But we do some of the "do_wakeup()" handling outside the pipe lock now
on the reader side

        __pipe_unlock(pipe);

        /* Signal writers asynchronously that there is more room. */
        if (do_wakeup) {
                wake_up_interruptible_poll(&pipe->wait, EPOLLOUT | EPOLLWRNORM);
                kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
        }

However, that isn't new to this series _either_, so I don't think
that's it. It does wake up things inside the lock _too_ if it ended up
emptying a whole buffer.

So it could be triggered by timing and behavior changes, but I doubt
this pipe_wait() thing is it either. The fact that it bisects to the
thing that changes things to use head/tail pointers makes me think
there's some other incorrect update or comparison somewhere.

That said, "pipe_wait()" is an abomination. It should use a proper
wait condition and use wait_event(), but the code predates all of
that. I suspect pipe_wait() goes back to the dark ages with the BKL
and no actual races between kernel code.

               Linus