Re: [PATCH tip/core/rcu 1/4] srcu: Fix __call_srcu()/process_srcu() datarace

[Joel Fernandes <joel@joelfernandes.org>]

On Mon, Feb 17, 2020 at 06:11:04PM +0100, Peter Zijlstra wrote:
> 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.

Oh ok, I understand what you mean now. Thank you for clarification!

thanks,

 - Joel