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
