On Tue, Feb 25, 2025 at 04:20:07PM -0500, Joel Fernandes wrote:
> On Tue, Feb 25, 2025 at 09:54:29AM -0800, Paul E. McKenney wrote:
> > On Tue, Feb 25, 2025 at 08:11:11AM -0800, Boqun Feng wrote:
> [...]
> > > > > These passed other than a KCSAN complaint involving
> > > > > rcu_preempt_deferred_qs_handler() and rcu_read_unlock_special().
> > > > > This looks like the plain C-language writes to ->defer_qs_iw_pending.
> > > > >
> > > > > My guess is that this is low probability, despite having happened
> > > > > twice,
> > > > > and that it happens when rcu_read_unlock_special() is interrupted,
> > > > > resulting in rcu_preempt_deferred_qs_handler() being invoked as an
> > > > > IRQ-work handler. Keeping in mind that RCU runs KCSAN so as to locate
> > > > > data races between task and handler on the same CPU.
> > > > >
> > > > > Thoughts?
> > > > >
> > > >
> > > > Do you have a KCSAN of this? Also this is not a regression, right?
> > > > Meaning you probably have seen this before? Anyway, it should be an easy
> > > > fix (just using READ_ONCE() and WRITE_ONCE()). I can send the fix out
> > > > and put it in.
> >
> > Here you go! And you are right, if it is a regression, it is from a
> > long time ago, though something more recent might have made it more
> > probable.
>
> In my opinion I probably wouldn't even call it a regression because the
> data-race is happening on a boolean element. If I am not mistaken, this is
> thus a false-positive and KCSAN has no way of silencing it?
You can still get in trouble with booleans. The usual example
is as follows:
bool x;
...
while (!x)
do_something();
In many cases, the compiler is free to transform that "while" loop
into this:
if (!x)
for (;;)
do_something();
Putting a READ_ONCE() in the original "while" condition prevents this
transformation.
Thanx, Paul
