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Date:   Mon, 17 Feb 2020 18:11:04 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     paulmck@kernel.org, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org,
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        oleg@redhat.com
Subject: Re: [PATCH tip/core/rcu 1/4] srcu: Fix __call_srcu()/process_srcu()
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On Mon, Feb 17, 2020 at 12:01:57PM -0500, Joel Fernandes wrote:

> Peter it sounds like you have a failure scenario in mind. Could you describe
> more if so?
> 
> I am curious if you were thinking of invented-stores issue here.
> 
> For educational purposes, I was trying to come up with an example where my
> compiler does something bad to code without WRITE_ONCE(). So far I only can
> reproduce a write-tearing example when write with an immediate value is split
> into 2 writes, like Will mentioned:
> http://lore.kernel.org/r/20190821103200.kpufwtviqhpbuv2n@willie-the-truck
> But that does not seem to apply to this code.

> > > -			snp->srcu_gp_seq_needed_exp = gpseq;
> > > +			WRITE_ONCE(snp->srcu_gp_seq_needed_exp, gpseq);

Yeah, store tearing. No sane compiler will actually do that, but it is
allowed to do random permutations of byte stores just to fuck with us.

WRITE_ONCE() disallows that.

In that case, the READ_ONCE()s could observe garbage and the compare
might accidentally report the wrong thing.