On Mon, Feb 17, 2020 at 10:16:16AM -0800, Paul E. McKenney wrote: > On Mon, Feb 17, 2020 at 01:38:51PM +0100, Peter Zijlstra wrote: > > On Fri, Feb 14, 2020 at 04:25:18PM -0800, paulmck@xxxxxxxxxx wrote: > > > From: "Paul E. McKenney" <paulmck@xxxxxxxxxx> > > > > > > The RCU tasks list of callbacks, rcu_tasks_cbs_head, is sampled locklessly > > > by rcu_tasks_kthread() when waiting for work to do. This commit therefore > > > applies READ_ONCE() to that lockless sampling and WRITE_ONCE() to the > > > single potential store outside of rcu_tasks_kthread. > > > > > > This data race was reported by KCSAN. Not appropriate for backporting > > > due to failure being unlikely. > > > > What failure is possible here? AFAICT this is (again) one of them > > load-complare-against-constant-discard patterns that are impossible to > > mess up. > > First, please keep in mind that this is RCU code. Rather uncomplicated > for RCU, to be sure, but still RCU code. > > The failure modes are thus as follows: > > o I produce a patch for which KCSAN gives a legitimate warning, > but this warning is obscured by a pile of other warnings. > Yes, we should continue improving KCSAN's ability to adapt > to the users desired compiler-optimization risk level, but > in RCU's case that risk level is set quite low. > > In RCU, what others are calling false positives are therefore > addressed. Yes, this does cost me a bit of work, but it is > trivial compared to the work required to track down a real bug. > > o Someone optimizes or otherwise changes the wait/wakeup code, > which inadvertently gives the compiler more scope for mischief. > > In short, within RCU, I am handling all KCSAN complaints. This is looking > to be an extremely inexpensive insurance policy for RCU. Other subsystems > are of course free to make their own tradeoffs, and subsystems having > less-aggressive concurrency control might be well-advised to take a > different path than the one I am taking. I just took offence at the Changelog wording. It seems to suggest there actually is a problem, there is not.
