Linux-Fsdevel Archive on lore.kernel.org
 help / color / Atom feed
From: Eric Dumazet <edumazet@google.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alan Stern <stern@rowland.harvard.edu>,
	Marco Elver <elver@google.com>,
	Eric Dumazet <eric.dumazet@gmail.com>,
	syzbot <syzbot+3ef049d50587836c0606@syzkaller.appspotmail.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Andrea Parri <parri.andrea@gmail.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>
Subject: Re: KCSAN: data-race in __alloc_file / __alloc_file
Date: Mon, 11 Nov 2019 09:52:41 -0800
Message-ID: <CANn89i+OBZOq-q4GWAxKVRau6nHYMo3v4y-c1vUb_O8nvra1RQ@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wjp6yR-gBNYXPzrHQHq+wX_t6WfwrF_S3EEUq9ccz3vng@mail.gmail.com>

On Mon, Nov 11, 2019 at 8:51 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Mon, Nov 11, 2019 at 7:51 AM Alan Stern <stern@rowland.harvard.edu> wrote:
> >
> > I dislike the explicit annotation approach, because it shifts the
> > burden of proving correctness from the automatic verifier to the
> > programmer.
>
> Yes.
>
> However, sometimes explicit annotations are very useful as
> documentation and as showing of intent even if they might not change
> behavior or code generation.
>
> But they generally should never _replace_ checking - in fact, the
> annotations themselves should hopefully be checked for correctness
> too.
>
> So a good annotation would implicitly document intent, but it should
> also be something that we can check being true, so that we also have
> the check that reality actually _matches_ the intent too. Because
> misleading and wrong documentation is worse than no documentation at
> all.
>
> Side note: an example of a dangerous annotation is the one that Eric
> pointed out, where a 64-bit read in percpu_counter_read_positive()
> could be changed to READ_ONCE(), and we would compile it cleanly, but
> on 32-bit it wouldn't actually be atomic.
>
> We at one time tried to actually verify that READ/WRITE_ONCE() was
> done only on types that could actually be accessed atomically (always
> ignoring alpha because the pain is not worth it), but it showed too
> many problems.
>
> So now we silently accept things that aren't actually atomic. We do
> access them "once" in the sense that we don't allow the compiler to
> reload it, but it's not "once" in the LKMM sense of one single value.
>
> That's ok for some cases. But it's actually a horrid horrid thing from
> a documentation standpoint, and I hate it, and it's dangerous.
>
>                 Linus

I was hoping to cleanup the 'easy cases'  before looking at more serious issues.

But it looks like even the ' easy cases'  are not that easy.

Now I wonder what to do with the ~400 KCSAN reports sitting in
pre-moderation queue.

  reply index

Thread overview: 67+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CAHk-=wjB61GNmqpX0BLA5tpL4tsjWV7akaTc2Roth7uGgax+mw@mail.gmail.com>
2019-11-10 16:09 ` Alan Stern
2019-11-10 19:10   ` Marco Elver
2019-11-11 15:51     ` Alan Stern
2019-11-11 16:51       ` Linus Torvalds
2019-11-11 17:52         ` Eric Dumazet [this message]
2019-11-11 18:04           ` Linus Torvalds
2019-11-11 18:31             ` Eric Dumazet
2019-11-11 18:44               ` Eric Dumazet
2019-11-11 19:00                 ` Linus Torvalds
2019-11-11 19:13                   ` Eric Dumazet
2019-11-11 20:43                     ` Linus Torvalds
2019-11-11 20:46                       ` Linus Torvalds
2019-11-11 21:53                         ` Eric Dumazet
2019-11-11 23:51                   ` Linus Torvalds
2019-11-12 16:50                     ` Kirill Smelkov
2019-11-12 17:23                       ` Linus Torvalds
2019-11-12 17:36                         ` Linus Torvalds
2019-11-17 18:56                           ` Kirill Smelkov
2019-11-17 19:20                             ` Linus Torvalds
2019-11-11 18:50               ` Linus Torvalds
2019-11-11 18:59                 ` Marco Elver
2019-11-11 18:59                 ` Eric Dumazet
2019-11-10 19:12   ` Linus Torvalds
2019-11-10 19:20     ` Linus Torvalds
2019-11-10 20:44       ` Paul E. McKenney
2019-11-10 21:10         ` Linus Torvalds
2019-11-10 21:31           ` Paul E. McKenney
2019-11-11 14:17         ` Marco Elver
2019-11-11 14:31           ` Paul E. McKenney
2019-11-11 15:10             ` Marco Elver
2019-11-13  0:25               ` Paul E. McKenney
2019-11-12 19:14     ` Alan Stern
2019-11-12 19:47       ` Linus Torvalds
2019-11-12 20:29         ` Alan Stern
2019-11-12 20:58           ` Linus Torvalds
2019-11-12 21:13             ` Linus Torvalds
2019-11-12 22:05               ` Marco Elver
2019-11-12 21:48             ` Alan Stern
2019-11-12 22:07               ` Eric Dumazet
2019-11-12 22:44                 ` Alexei Starovoitov
2019-11-12 23:17                   ` Eric Dumazet
2019-11-12 23:40                     ` Linus Torvalds
2019-11-13 15:00                       ` Marco Elver
2019-11-13 16:57                         ` Linus Torvalds
2019-11-13 21:33                           ` Marco Elver
2019-11-13 21:50                             ` Alan Stern
2019-11-13 22:48                               ` Marco Elver
2019-11-08 13:16 syzbot
2019-11-08 13:28 ` Eric Dumazet
2019-11-08 17:01   ` Linus Torvalds
2019-11-08 17:22     ` Eric Dumazet
2019-11-08 17:38       ` Linus Torvalds
2019-11-08 17:53         ` Eric Dumazet
2019-11-08 17:55           ` Eric Dumazet
2019-11-08 18:02             ` Eric Dumazet
2019-11-08 18:12               ` Linus Torvalds
2019-11-08 20:30             ` Linus Torvalds
2019-11-08 20:53               ` Eric Dumazet
2019-11-08 21:36                 ` Linus Torvalds
2019-11-08 18:05           ` Linus Torvalds
2019-11-08 18:15             ` Marco Elver
2019-11-08 18:40               ` Linus Torvalds
2019-11-08 19:48                 ` Marco Elver
2019-11-08 20:26                   ` Linus Torvalds
2019-11-08 21:57                     ` Alan Stern
2019-11-08 22:06                       ` Linus Torvalds
2019-11-09 23:08                         ` Alan Stern

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=CANn89i+OBZOq-q4GWAxKVRau6nHYMo3v4y-c1vUb_O8nvra1RQ@mail.gmail.com \
    --to=edumazet@google.com \
    --cc=akiyks@gmail.com \
    --cc=elver@google.com \
    --cc=eric.dumazet@gmail.com \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=parri.andrea@gmail.com \
    --cc=paulmck@kernel.org \
    --cc=stern@rowland.harvard.edu \
    --cc=syzbot+3ef049d50587836c0606@syzkaller.appspotmail.com \
    --cc=syzkaller-bugs@googlegroups.com \
    --cc=torvalds@linux-foundation.org \
    --cc=viro@zeniv.linux.org.uk \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-Fsdevel Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-fsdevel/0 linux-fsdevel/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-fsdevel linux-fsdevel/ https://lore.kernel.org/linux-fsdevel \
		linux-fsdevel@vger.kernel.org
	public-inbox-index linux-fsdevel

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-fsdevel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git