On Tue, 17 May, at 04:11:09AM, Yuyang Du wrote: > On Mon, May 16, 2016 at 10:46:38AM +0100, Matt Fleming wrote: > > > > No because if someone calls rq_clock() immediately after __schedule(), > > or even immediately after we clear RQCF_ACT_SKIP in __schedule(), we > > should trigger a warning since the clock has not actually been > > updated. > > Well, I don't know how concurrent it can be, but aren't both update > and read synchronized by rq->lock? So I don't understand the latter > case, and the former should be addressed (missing its own update?).
I'm not talking about concurrency; when I said "someone" above, I was referring to code. So, if the code looks like the following, either now or in the future, static void __schedule(bool preempt) { ... /* Clear RQCF_ACT_SKIP */ rq->clock_update_flags = 0; ... delta = rq_clock(); } I would expect to see a warning triggered, because we've read the rq clock outside of the code area where we know it's safe to do so without a clock update. The solution for that bug may be as simple as rearranging the code, delta = rq_clock(); ... rq->clock_update_flags = 0; but we definitely want to catch such bugs in the first instance.