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>

diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
index 1522c7ae31b0c239901237569bad5cbb4ec7b02e..dd1c6aacbaf9d8e5fc7ad220a3a8238f00a14188 100644 (file)
--- 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);
 }