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

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Date:   Tue, 28 Jan 2020 17:56:55 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marco Elver <elver@...gle.com>
Cc:     Qian Cai <cai@....pw>, Will Deacon <will@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "paul E. McKenney" <paulmck@...nel.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 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.

> 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?

> 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 :-)