On Mon, Dec 07 2020 at 19:19, Marco Elver wrote: > On Mon, 7 Dec 2020 at 18:46, Thomas Gleixner <t...@linutronix.de> wrote: >> On Mon, Dec 07 2020 at 13:09, Peter Zijlstra wrote: >> > I prefer the form: >> > >> > if (data_race(tick_do_timer_cpu == TICK_DO_TIMER_BOOT)) { >> > >> > But there doesn't yet seem to be sufficient data_race() usage in the >> > kernel to see which of the forms is preferred. Do we want to bike-shed >> > this now and document the outcome somewhere? >> >> Yes please before we get a gazillion of patches changing half of them >> half a year from now. > > That rule should be as simple as possible. The simplest would be: > "Only enclose the smallest required expression in data_race(); keep > the number of required data_race() expressions to a minimum." (=> want > least amount of code inside data_race() with the least number of > data_race()s). > > In the case here, that'd be the "if (data_race(tick_do_timer_cpu) == > ..." variant. > > Otherwise there's the possibility that we'll end up with accesses > inside data_race() that we hadn't planned for. For example, somebody > refactors some code replacing constants with variables. > > I currently don't know what the rule for Peter's preferred variant > would be, without running the risk of some accidentally data_race()'d > accesses.
I agree. Lets keep it simple and have the data_race() only covering the actual access to the racy variable, struct member. The worst case we could end up with would be if (data_race(A) == data_race(B)) which would still be clearly isolated. The racy part is not the comparison, it's the accesses which can cause random results for the comparison. Thanks, tglx