Re: arm-linux-gnueabihf-ld: kernel/rcu/update.o:update.c:(.text+0x1cc4): more undefined references to `__bad_cmpxchg' follow

[Paul E. McKenney]

On Thu, May 30, 2024 at 09:37:21AM -0700, Paul E. McKenney wrote:
> 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@xxxxxxxxxx>
> > >> 
> > >> 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.

And for an untested first attempt at a fix.

What did I mess up this time?  ;-)

							Thanx, Paul

------------------------------------------------------------------------

commit 6e43483dd111cf1be58b02a45d0ca729ca2634ba
Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
Date:   Thu May 30 10:11:31 2024 -0700

    ARM: Emulate one-byte cmpxchg
    
    Use the new cmpxchg_emu_u8() to emulate one-byte cmpxchg() on ARM systems
    with ARCH < ARMv6K.
    
    Reported-by: Mark Brown <broonie@xxxxxxxxxx>
    Closes: https://lore.kernel.org/all/54798f68-48f7-4c65-9cba-47c0bf175143@xxxxxxxxxxxxx/
    Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
    Closes: https://lore.kernel.org/all/CA+G9fYuZ+pf6p8AXMZWtdFtX-gbG8HMaBKp=XbxcdzA_QeLkxQ@xxxxxxxxxxxxxx/
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
    Cc: "Russell King (Oracle)" <rmk+kernel@xxxxxxxxxxxxxxx>
    Cc: Arnd Bergmann <arnd@xxxxxxxx>
    Cc: Andrew Davis <afd@xxxxxx>
    Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
    Cc: Linus Walleij <linus.walleij@xxxxxxxxxx>
    Cc: Eric DeVolder <eric.devolder@xxxxxxxxxx>
    Cc: Rob Herring <robh@xxxxxxxxxx>
    Cc: <linux-arm-kernel@xxxxxxxxxxxxxxxxxxx>

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index ee5115252aac4..a867a7d967aa5 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -34,6 +34,7 @@ config ARM
 	select ARCH_MIGHT_HAVE_PC_PARPORT
 	select ARCH_OPTIONAL_KERNEL_RWX if ARCH_HAS_STRICT_KERNEL_RWX
 	select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT if CPU_V7
+	select ARCH_NEED_CMPXCHG_1_EMU if CPU_V6
 	select ARCH_SUPPORTS_ATOMIC_RMW
 	select ARCH_SUPPORTS_CFI_CLANG
 	select ARCH_SUPPORTS_HUGETLBFS if ARM_LPAE
diff --git a/arch/arm/include/asm/cmpxchg.h b/arch/arm/include/asm/cmpxchg.h
index 44667bdb4707a..fd9b99f4dca46 100644
--- a/arch/arm/include/asm/cmpxchg.h
+++ b/arch/arm/include/asm/cmpxchg.h
@@ -162,7 +162,11 @@ static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
 	prefetchw((const void *)ptr);
 
 	switch (size) {
-#ifndef CONFIG_CPU_V6	/* min ARCH >= ARMv6K */
+#ifdef CONFIG_CPU_V6	/* min ARCH >= ARMv6K */
+	case 1:
+		oldval = cmpxchg_emu_u8((volatile u8 *)ptr, old, new);
+		break;
+#else
 	case 1:
 		do {
 			asm volatile("@ __cmpxchg1\n"