[PATCH] ARM: change definition of cpu_relax() for ARM11MPCore

Will Deacon will.deacon at arm.com
Tue Mar 9 11:06:08 EST 2010

The cpu_relax() macro is often used in the body of busy-wait loops to ensure
that the variable being spun on is re-loaded for each iteration. On the
ARM11MPCore processor [where loads are prioritised over stores], spinning in
such a loop will prevent the write buffer from draining. If a write contained
in the write buffer indirectly affects the variable being spun on, there is a
potential for deadlock. This deadlock is experienced when executing the KGDB
testsuite on an SMP ARM11MPCore configuration.

This patch changes the definition of cpu_relax() to smp_mb() for ARMv6 cores,
forcing a flushing of the write buffer on SMP systems. If the Kernel is not
compiled for SMP support, this will expand to a barrier() as before.

Cc: KGDB Mailing List <kgdb-bugreport at lists.sourceforge.net>
Acked-by: Catalin Marinas <catalin.marinas at arm.com>
Signed-off-by: Will Deacon <will.deacon at arm.com>
 arch/arm/include/asm/processor.h |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/arch/arm/include/asm/processor.h b/arch/arm/include/asm/processor.h
index 6a89567..7bed3da 100644
--- a/arch/arm/include/asm/processor.h
+++ b/arch/arm/include/asm/processor.h
@@ -91,7 +91,11 @@ extern void release_thread(struct task_struct *);
 unsigned long get_wchan(struct task_struct *p);
+#if __LINUX_ARM_ARCH__ == 6
+#define cpu_relax()			smp_mb()
 #define cpu_relax()			barrier()
  * Create a new kernel thread

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