On Thu, May 30, 2024 at 03:27:58PM +0200, Arnd Bergmann wrote:
On Thu, May 30, 2024, at 14:26, Russell King (Oracle) wrote:
On Thu, May 30, 2024 at 05:50:52PM +0530, Naresh Kamboju wrote:
The arm builds failed on Linux next with gcc-13 and clang-18.
Config: arm imx_v6_v7_defconfig - failed
Reported-by: Linux Kernel Functional Testing lkft@linaro.org
Build log:
arm-linux-gnueabihf-ld: kernel/rcu/update.o: in function `rcu_trc_cmpxchg_need_qs': update.c:(.text+0x348): undefined reference to `__bad_cmpxchg' arm-linux-gnueabihf-ld: kernel/rcu/update.o: in function `rcu_read_unlock_trace_special': update.c:(.text+0x41c): undefined reference to `__bad_cmpxchg' arm-linux-gnueabihf-ld: kernel/rcu/update.o: in function `trc_read_check_handler': update.c:(.text+0x4b0): undefined reference to `__bad_cmpxchg' arm-linux-gnueabihf-ld: kernel/rcu/update.o: in function `trc_inspect_reader': update.c:(.text+0x1518): undefined reference to `__bad_cmpxchg' arm-linux-gnueabihf-ld: update.c:(.text+0x1548): undefined reference to `__bad_cmpxchg' arm-linux-gnueabihf-ld: kernel/rcu/update.o:update.c:(.text+0x1cc4): more undefined references to `__bad_cmpxchg' follow make[3]: *** [/builds/linux/scripts/Makefile.vmlinux:34: vmlinux] Error 1
You get a reference to __bad_cmpxchg() when someone uses cmpxchg() on a datatype that there are no CPU instructions to perform the atomic compare-and-exchange.
As we support 8-bit, 16-bit and 32-bit, I would expect that we're now seeing cmpxchg() being used on 64-bit values, which are unsupportable on 32-bit Arm.
A 64-bit cmpxchg() would fail on most 32-bit architectures, other than armv6k+ and i686+. Since this one fails on an armv6 (non-6k) build, it's probably the added 8-bit cmpxchg() that Paul talked about using for RCU, though I don't see that in linux-next here.
I htink we need to use the emulated cmpxchg8() in __cmpxchg() for this case, just like we do on other architectures that only have word size atomics.
Thank you all for your testing and analysis!
I will fix this, and also pull the 8046f3005015 ("rcu-tasks: Remove open-coded one-byte cmpxchg() emulation") commit out of my -next pile in the meantime.
Thanx, Paul