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Date:   Tue, 3 Mar 2020 20:05:15 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Qian Cai <cai@lca.pw>, elver@google.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH -next] lib: disable KCSAN for XArray
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 <20200304033329.GZ29971@bombadil.infradead.org>
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On Tue, Mar 03, 2020 at 07:33:29PM -0800, Matthew Wilcox wrote:
> On Tue, Mar 03, 2020 at 10:15:51PM -0500, Qian Cai wrote:
> > Functions like xas_find_marked(), xas_set_mark(), and xas_clear_mark()
> > could happen concurrently result in data races, but those operate only
> > on a single bit that are pretty much harmless. For example,
> 
> Those aren't data races.  The writes are protected by a spinlock and the
> reads by the RCU read lock.  If the tool can't handle RCU protection,
> it's not going to be much use.

Would KCSAN's ASSERT_EXCLUSIVE_BITS() help here?

If not, you lost me on this one.  RCU readers don't exclude lock-based
writers.

RCU readers -do- exclude pre-insertion initialization on the one hand,
and those post-removal accesses that follow a grace period, but only
if that grace period starts after the removal.  In addition, the
accesses due to rcu_dereference(), rcu_assign_pointer(), and similar
are guaranteed to work even if they are concurrent.

Or am I missing something subtle here?

That said, you are permitted to define "data race" for your subsystem
by choosing KCSAN settings, up to and including keeping KCSAN out
entirely.

							Thanx, Paul