From: Linus Torvalds <torvalds@linux-foundation.org>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: syzbot <syzbot+3ef049d50587836c0606@syzkaller.appspotmail.com>,
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>,
Eric Dumazet <edumazet@google.com>
Subject: Re: KCSAN: data-race in __alloc_file / __alloc_file
Date: Fri, 8 Nov 2019 09:01:31 -0800 [thread overview]
Message-ID: <CAHk-=whh5bcxCecEL5Fy4XvQjgBTJ9uqvyp7dW=CLU6VNxS9iA@mail.gmail.com> (raw)
In-Reply-To: <6bddae34-93df-6820-0390-ac18dcbf0927@gmail.com>
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.
Linus
next prev parent reply other threads:[~2019-11-08 17:02 UTC|newest]
Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-08 13:16 KCSAN: data-race in __alloc_file / __alloc_file syzbot
2019-11-08 13:28 ` Eric Dumazet
2019-11-08 17:01 ` Linus Torvalds [this message]
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
[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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