Re: [PATCH 2/9] rcu: Fixup noinstr warnings

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, Jun 04, 2020 at 10:05:12AM +0200, Peter Zijlstra wrote:
> On Wed, Jun 03, 2020 at 08:34:09PM -0700, Paul E. McKenney wrote:
> > On Wed, Jun 03, 2020 at 07:13:20PM +0200, Peter Zijlstra wrote:
> > > On Wed, Jun 03, 2020 at 09:46:00AM -0700, Paul E. McKenney wrote:
> 
> > > > > @@ -313,7 +313,7 @@ static __always_inline bool rcu_dynticks
> > > > >  {
> > > > >  	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
> > > > >  
> > > > > -	return !(atomic_read(&rdp->dynticks) & RCU_DYNTICK_CTRL_CTR);
> > > > > +	return !(arch_atomic_read(&rdp->dynticks) & RCU_DYNTICK_CTRL_CTR);
> > > 
> > > The above is actually instrumented by KCSAN, due to arch_atomic_read()
> > > being a READ_ONCE() and it now understanding volatile.
> > > 
> > > > Also instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks)) as
> > 
> > Right, this should instead be instrument_read(...).
> > 
> > Though if KCSAN is unconditionally instrumenting volatile, how does
> > this help?  Or does KCSAN's instrumentation of volatile somehow avoid
> > causing trouble?
> 
> As Marco already explained, when used inside noinstr no instrumentation
> will be emitted, when used outside noinstr it will emit the right
> instrumentation.
> 
> > > > o	In theory in rcu_irq_exit_preempt(), but as this generates code
> > > > 	only in lockdep builds, it might not be worth worrying about.
> > > > 
> > > > o	Ditto for rcu_irq_exit_check_preempt().
> > > > 
> > > > o	Ditto for __rcu_irq_enter_check_tick().
> > > 
> > > Not these, afaict they're all the above arch_atomic_read(), which is
> > > instrumented due to volatile in these cases.
> 
> I this case, the above call-sites are all not noinstr (double negative!)
> and will thus cause instrumentation to be emitted.
> 
> This is all a 'special' case for arch_atomic_read() (and _set()),
> because they're basically READ_ONCE() (and WRITE_ONCE() resp.). The
> normal atomics are asm() and it doesn't do anything for those (although
> I suppose clang could, since it has this internal assembler to parse the
> inline asm, but afaiu that's not something GCC ever wants to do).

Got it, and I had missed the inlining.

Again, commenting this will be interesting.  And your earlier comment
about the compiler refusing to inline now makes sense...

							Thanx, Paul