On Tue, Nov 17, 2020 at 08:09:22PM -0500, Steven Rostedt wrote: > On Tue, 17 Nov 2020 16:42:44 -0800 > Matt Mullins <mmullins@xxxxxxx> wrote: > > > > > Indeed with a stub function, I don't see any need for READ_ONCE/WRITE_ONCE. > > > > I'm not sure if this is a practical issue, but without WRITE_ONCE, can't > > the write be torn? A racing __traceiter_ could potentially see a > > half-modified function pointer, which wouldn't work out too well. > > This has been discussed before, and Linus said: > > "We add READ_ONCE and WRITE_ONCE annotations when they make sense. Not > because of some theoretical "compiler is free to do garbage" > arguments. If such garbage happens, we need to fix the compiler" > > https://lore.kernel.org/lkml/CAHk-=wi_KeD1M-_-_SU_H92vJ-yNkDnAGhAS=RR1yNNGWKW+aA@xxxxxxxxxxxxxx/ I have to ask... Did the ARM compilers get fixed? As of a few months ago, they would tear stores of some constant values. > > This was actually my gut instinct before I wrote the __GFP_NOFAIL > > instead -- currently that whole array's memory ordering is provided by > > RCU and I didn't dive deep enough to evaluate getting too clever with > > atomic modifications to it. > > The pointers are always going to be the architecture word size (by > definition), and any compiler that tears a write of a long is broken. But yes, if the write is of a non-constant pointer, the compiler does have less leverage. Thanx, Paul
