linux-kernel - Re: KCSAN: data-race in __alloc_file / __alloc

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Message-ID: <Pine.LNX.4.44L0.1911121639540.1567-100000@iolanthe.rowland.org>
Date:   Tue, 12 Nov 2019 16:48:38 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Marco Elver <elver@...gle.com>, Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        syzbot <syzbot+3ef049d50587836c0606@...kaller.appspotmail.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrea Parri <parri.andrea@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>
Subject: Re: KCSAN: data-race in __alloc_file / __alloc_file

On Tue, 12 Nov 2019, Linus Torvalds wrote:

> Honestly, my preferred model would have been to just add a comment,
> and have the reporting tool know to then just ignore it. So something
> like
> 
> +               // Benign data-race on min_flt
>                 tsk->min_flt++;
>                 perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1, regs, address);
> 
> for the case that Eric mentioned - the tool would trigger on
> "data-race", and the rest of the comment could/should be for humans.
> Without making the code uglier, but giving the potential for a nice
> leghibl.e explanation instead of a completely illegible "let's
> randomly use WRITE_ONCE() here" or something like that.

Just to be perfectly clear, then:

Your feeling is that we don't need to tell the compiler anything at all 
about these races, because if a compiler generates code that is 
non-robust against such things then you don't want to use it for the 
kernel.

And as a corollary, the only changes you want to make to the source
code are things that tell KCSAN not to worry about these races when
they occur.

Right?

> +		// Benign data-race on min_flt
> 		tsk->min_flt++;
> 		perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1, regs, address);

I suggest grouping the accesses into classes somehow, and telling KCSAN
that races between accesses in the same class are okay but racing
accesses in different classes should trigger a warning.  That would
give the tool a better chance of finding genuine races.

Alan Stern