Re: KCSAN: data-race in __alloc_file / __alloc_file

From: "Paul E. McKenney" <paulmck@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alan Stern <stern@rowland.harvard.edu>,
	Marco Elver <elver@google.com>,
	Eric Dumazet <edumazet@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>,
	LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>
Subject: Re: KCSAN: data-race in __alloc_file / __alloc_file
Date: Sun, 10 Nov 2019 13:31:38 -0800	[thread overview]
Message-ID: <20191110213138.GH2865@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <CAHk-=wjm40mcf7tk9DZQXd=dftZw_VpmE837c7pTZ1_cR+t4Mw@mail.gmail.com>

On Sun, Nov 10, 2019 at 01:10:39PM -0800, Linus Torvalds wrote:
> On Sun, Nov 10, 2019 at 12:44 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > But will "one size fits all" be practical and useful?
> 
> Oh, I do agree that if KCSAN has some mode where it says "I'll ignore
> repeated writes with the same value" (or whatever), it could/should
> likely be behind some flag.
> 
> I don't think it should be a subsystem flag, though. More of a "I'm
> willing to actually analyze and ignore false positives" flag. Because
> I don't think it's so much about the code, as it is about the person
> who looks at the results.
> 
> For example, we're already getting push-back from people on some of
> the KCSAN-inspired patches. If we have people sending patches to add
> READ_ONCE/WRITE_ONCE to random places to shut up KCSAN reports, I
> don't think that's good.
> 
> But if we have people who _work_ on memory ordering issues etc, and
> want to see a strict mode, knowing there are false positives and able
> to handle them, that's a completely different thing..
> 
> No?

Understood on the pushback!  And I especially agree that it is bad to
automatically add *_ONCE() just to shut up KCSAN.  For one thing, doing
that inconveniences people later on who might want to take a closer look.

As long as I can get the full-up reports for RCU.  And as long as the
others who want the full-up reports can also get them.  ;-)

And agreed, if the results are adjusted based on who is processing them,
that should be good.

							Thanx, Paul