ARM: 7669/1: keep __my_cpu_offset consistent with generic one

Commit 14318efb(ARM: 7587/1: implement optimized percpu variable access)
introduces arm's __my_cpu_offset to optimize percpu vaiable access,
which really works well on hackbench, but will cause __my_cpu_offset
to return garbage value before it is initialized in cpu_init() called
by setup_arch, so accessing percpu variable before setup_arch may cause
kernel hang. But generic __my_cpu_offset always returns zero before
percpu area is brought up, and won't hang kernel.

So the patch tries to clear __my_cpu_offset on boot CPU early
to avoid boot hang.

At least now percpu variable is accessed by lockdep before
setup_arch(), and enabling CONFIG_LOCK_STAT or CONFIG_DEBUG_LOCKDEP
can trigger kernel hang.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
(cherry picked from commit 9394c1c65e61eb6f4c1c99f342b49e451ec337b6)

1 files changed, 7 insertions, 0 deletions

diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
index b4b1d397592b..0cde326f5542 100644
--- a/arch/arm/kernel/setup.c
+++ b/arch/arm/kernel/setup.c
@@ -456,6 +456,13 @@ void __init smp_setup_processor_id(void)
         for (i = 1; i < nr_cpu_ids; ++i)
                 cpu_logical_map(i) = i == cpu ? 0 : i;
 
+        /*
+         * clear __my_cpu_offset on boot CPU to avoid hang caused by
+         * using percpu variable early, for example, lockdep will
+         * access percpu variable inside lock_release
+         */
+        set_my_cpu_offset(0);
+
         printk(KERN_INFO "Booting Linux on physical CPU 0x%x\n", mpidr);
 }