On 2/25/2025 4:53 PM, Paul E. McKenney wrote: > 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. True, thanks for clarifying. I will be a bit more annoying and say that in rcu_read_unlock_special(), there is no such looping transformation possible though AFAICS. The test is an if() block. But this is beyond KCSAN's ability to analyze I guess. Thanks! - Joel
