linux-kernel - Re: [PATCH] locking/osq_lock: fix a data race in osq_wait

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Message-ID: <20200129002253.GT2935@paulmck-ThinkPad-P72>
Date:   Tue, 28 Jan 2020 16:22:53 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marco Elver <elver@...gle.com>, Qian Cai <cai@....pw>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [PATCH] locking/osq_lock: fix a data race in osq_wait_next

On Tue, Jan 28, 2020 at 05:56:55PM +0100, Peter Zijlstra wrote:
> On Tue, Jan 28, 2020 at 12:46:26PM +0100, Marco Elver wrote:
> 
> > > Marco, any thought on improving KCSAN for this to reduce the false
> > > positives?
> > 
> > Define 'false positive'.
> 
> I'll use it where the code as written is correct while the tool
> complains about it.

I could be wrong, but I would guess that Marco is looking for something
a little less subjective and a little more specific.  ;-)

> > From what I can tell, all 'false positives' that have come up are data
> > races where the consequences on the behaviour of the code is
> > inconsequential. In other words, all of them would require
> > understanding of the intended logic of the code, and understanding if
> > the worst possible outcome of a data race changes the behaviour of the
> > code in such a way that we may end up with an erroneously behaving
> > system.
> > 
> > As I have said before, KCSAN (or any data race detector) by definition
> > only works at the language level. Any semantic analysis, beyond simple
> > rules (such as ignore same-value stores) and annotations, is simply
> > impossible since the tool can't know about the logic that the
> > programmer intended.
> > 
> > That being said, if there are simple rules (like ignore same-value
> > stores) or other minimal annotations that can help reduce such 'false
> > positives', more than happy to add them.
> 
> OK, so KCSAN knows about same-value-stores? If so, that ->cpu =
> smp_processor_id() case really doesn't need annotation, right?

If smp_processor_id() returns the value already stored in ->cpu,
I believe that the default KCSAN setup refrains from complaining.

Which reminds me, I need to disable this in my RCU runs.  If I create a
bug that causes me to unknowingly access something that is supposed to
be CPU-private from the wrong CPU, I want to know about it.

> > What to do about osq_lock here? If people agree that no further
> > annotations are wanted, and the reasoning above concludes there are no
> > bugs, we can blacklist the file. That would, however, miss new data
> > races in future.
> 
> I'm still hoping to convince you that the other case is one of those
> 'simple-rules' too :-)

On this I must defer to Marco.

							Thanx, Paul