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From: Eric Dumazet <edumazet@google.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Eric Dumazet <eric.dumazet@gmail.com>,
	syzbot <syzbot+3ef049d50587836c0606@syzkaller.appspotmail.com>,
	Marco Elver <elver@google.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>
Subject: Re: KCSAN: data-race in __alloc_file / __alloc_file
Date: Fri, 8 Nov 2019 09:22:41 -0800
Message-ID: <CANn89iK9mTJ4BN-X3MeSx5LGXGYafXkhZyaUpdXDjVivTwA6Jg@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=whh5bcxCecEL5Fy4XvQjgBTJ9uqvyp7dW=CLU6VNxS9iA@mail.gmail.com>

On Fri, Nov 8, 2019 at 9:01 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Fri, Nov 8, 2019 at 5:28 AM Eric Dumazet <eric.dumazet@gmail.com> wrote:
> >
> > Linus, what do you think of the following fix ?
>
> I think it's incredibly ugly.
>
> I realize that avoiding the cacheline dirtying might be worth it, but
> I'd like to see some indication that it actually matters and helps
> from a performance angle. We've already dirtied memory fairly close,
> even if it might not share a cacheline (that structure is randomized,
> we've touched - or will touch - 'cred->usage') too.
>
> Honestly, I don't think get_cred() is even in a hotpath. Most cred use
> just use the current cred that doesn't need the 'get'. So the
> optimization looks somewhat questionable - for all we know it just
> makes things worse.
>
> I also don't like using a "WRITE_ONCE()" without a reason for it. In
> this case, the only "reason" is that KCSAN special-cases that thing.
> I'd much rather have some other way to mark it.
>
> So it just looks hacky to me.
>
> I like that people are looking at KCSAN, but I get a very strong
> feeling that right now the workarounds for KCSAN false-positives are
> incredibly ugly, and not always appropriate.
>
> There is absolutely zero need for a WRITE_ONCE() in this case. The
> code would work fine if the compiler did the zero write fifty times,
> and re-ordered it wildly. We have a flag that starts out set, and we
> clear it.  There's really no "write-once" about it.
>

Ok, so what do you suggest next ?

Declare KCSAN useless because too many false positives ?

  reply index

Thread overview: 67+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
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 [this message]
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
     [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
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

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