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
