On Wed, 6 Mar 2024 11:01:55 -0800 Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > On Wed, 6 Mar 2024 at 10:53, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > > > > Now, are you OK with an addition of ADD_ONCE() and/or INC_ONCE()? So that we > > don't have to look at: > > > > WRITE_ONCE(a, READ_ONCE(a) + 1); > > > > ? > > In a generic header file under include/linux/? > > Absolutely not. The above is a completely broken operation. There is > no way in hell we should expose it as a general helper. > > So there is no way we'd add that kind of sh*t-for-brains operation in > (for example) our <asm/rwonce.h> header file next to the normal > READ/WRITE_ONCE defines. > > In some individual tracing C file where it has a comment above it how > it's braindamaged and unsafe and talking about why it's ok in that > particular context? Go wild. Note this has nothing to do with tracing. This thread is in RCU. I just happen to receive the same patch "fix" for my code. > > But honestly, I do not see when a ADD_ONCE() would ever be a valid > thing to do, and *if* it's a valid thing to do, why you'd do it with > READ_ONCE and WRITE_ONCE. > > If you don't care about races, just do a simple "++" and be done with > it. The end result is random. That was my feeling. But when I saw this going into RCU, I was thinking there was a more fundamental problem here. > > Adding a "ADD_ONCE()" macro doesn't make it one whit less random. It > just makes a broken concept even uglier. > > So honestly, I think the ADD_ONCE macro not only needs to be in some > tracing-specific C file, the comment needs to be pretty damn big too. > Because as a random number generator, it's not even a very good one. > So you need to explain *why* you want a particularly bad random number > generator in the first place. Again, this has nothing to do with tracing. The code here is solely in RCU. I did receive a patch in the tracing code, but that had to deal with wakeups of readers with respect to writers which is a common thing across the kernel and is not anything tracing specific. I wasn't about to take the patch to my code, but when I saw the same changes in RCU, then I thought this might be something I need to worry about. -- Steve
