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Date: Thu, 30 May 2024 13:26:35 +0100
From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Naresh Kamboju <naresh.kamboju@linaro.org>
Cc: open list <linux-kernel@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	lkft-triage@lists.linaro.org,
	Linux Regressions <regressions@lists.linux.dev>,
	rcu <rcu@vger.kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Dan Carpenter <dan.carpenter@linaro.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Joel Fernandes <joel@joelfernandes.org>, eeraj.upadhyay@kernel.org,
	John Ogness <john.ogness@linutronix.de>
Subject: Re: arm-linux-gnueabihf-ld:
 kernel/rcu/update.o:update.c:(.text+0x1cc4): more undefined references to
 `__bad_cmpxchg' follow
Message-ID: <Zlhwe5owmbzI3jJK@shell.armlinux.org.uk>
References: <CA+G9fYuZ+pf6p8AXMZWtdFtX-gbG8HMaBKp=XbxcdzA_QeLkxQ@mail.gmail.com>
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Sender: Russell King (Oracle) <linux@armlinux.org.uk>

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.

-- 
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