Hi Wilco, Thanks for your email.
On Tue, Sep 26, 2023 at 12:07 AM Wilco Dijkstra <wilco.dijks...@arm.com> wrote: > > Hi Ramana, > > >> __sync_val_compare_and_swap may be used on 128-bit types and either calls > >> the > >> outline atomic code or uses an inline loop. On AArch64 LDXP is only > >> atomic if > >> the value is stored successfully using STXP, but the current > >> implementations > >> do not perform the store if the comparison fails. In this case the value > >> returned > >> is not read atomically. > > > > IIRC, the previous discussions in this space revolved around the > > difficulty with the store writing to readonly memory which is why I > > think we went with LDXP in this form. > > That's not related to this patch - this fixes a serious atomicity bug that may > affect the Linux kernel since it uses the older sync primitives. Given that > LDXP > is not atomic on its own, you have to execute the STXP even in the failure > case. > Note that you can't rely on compare&swap not to write memory: load-exclusive > loops may either always write or avoid writes in the failure case if the load > is > atomic. CAS instructions always write. > I am aware of the capabilities of the architecture. > > Has something changed from then ? > > Yes, we now know that using locking atomics was a bad decision. Developers > actually require efficient and lock-free atomics. Since we didn't support > them, > many applications were forced to add their own atomic implementations using > hacky inline assembler. It also resulted in a nasty ABI incompatibility > between > GCC and LLVM. Yes - atomics are part of the ABI! I agree that atomics are part of the ABI. > > All that is much worse than worrying about a theoretical corner case that > can't happen in real applications - atomics only work on writeable memory > since their purpose is to synchronize reads with writes. I remember this to be the previous discussions and common understanding. https://gcc.gnu.org/legacy-ml/gcc/2016-06/msg00017.html and here https://gcc.gnu.org/legacy-ml/gcc-patches/2017-02/msg00168.html Can you point any discussion recently that shows this has changed and point me at that discussion if any anywhere ? I can't find it in my searches . Perhaps you've had the discussion some place to show it has changed. regards Ramana > > Cheers, > Wilco
