From: Linus Torvalds <torvalds@linux-foundation.org>
To: David Sterba <dsterba@suse.cz>,
David Howells <dhowells@redhat.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Rasmus Villemoes <linux@rasmusvillemoes.dk>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Peter Zijlstra <peterz@infradead.org>,
raven@themaw.net, Christian Brauner <christian@brauner.io>,
keyrings@vger.kernel.org, linux-usb@vger.kernel.org,
linux-block <linux-block@vger.kernel.org>,
LSM List <linux-security-module@vger.kernel.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Linux API <linux-api@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [GIT PULL] pipe: Notification queue preparation
Date: Thu, 5 Dec 2019 09:33:04 -0800 [thread overview]
Message-ID: <CAHk-=whw+R5GVQdpV6J_5afQ=76vtBPzBPRj6-zG1tnhT32Pag@mail.gmail.com> (raw)
In-Reply-To: <20191205172127.GW2734@suse.cz>
On Thu, Dec 5, 2019 at 9:22 AM David Sterba <dsterba@suse.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
next prev parent reply other threads:[~2019-12-05 17:33 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-25 22:39 [GIT PULL] pipe: Notification queue preparation David Howells
2019-11-30 23:05 ` pr-tracker-bot
2019-12-05 12:58 ` David Sterba
2019-12-05 13:56 ` David Howells
2019-12-05 17:12 ` Linus Torvalds
2019-12-05 17:24 ` Linus Torvalds
2019-12-05 17:21 ` David Sterba
2019-12-05 17:33 ` Linus Torvalds [this message]
2019-12-05 17:25 ` David Howells
2019-12-05 18:18 ` David Sterba
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