[PATCH] x86: serialize SMP boot's CMOS accesses on rtc_lock

["Jan Beulich" <JBeulich@novell.com>]

With CPU hotplug, there is a theoretical race between other CMOS
(namely RTC) accesses and those done in the SMP secondary processor
bringup path. I am unware of the problem having been noticed by anyone
in practice, but it would very likely be rather spurious and very hard
to reproduce. So to be on the safe side, acquire rtc_lock around those
accesses.

Signed-off-by: Jan Beulich <jbeulich@novell.com>

---
 arch/x86/include/asm/smpboot_hooks.h |    8 ++++++++
 1 file changed, 8 insertions(+)

--- 3.0-rc7/arch/x86/include/asm/smpboot_hooks.h
+++ 3.0-rc7-x86-rtc-lock-SMP/arch/x86/include/asm/smpboot_hooks.h
@@ -10,7 +10,11 @@ static inline void smpboot_clear_io_apic
 
 static inline void smpboot_setup_warm_reset_vector(unsigned long start_eip)
 {
+	unsigned long flags;
+
+	spin_lock_irqsave(&rtc_lock, flags);
 	CMOS_WRITE(0xa, 0xf);
+	spin_unlock_irqrestore(&rtc_lock, flags);
 	local_flush_tlb();
 	pr_debug("1.\n");
 	*((volatile unsigned short *)phys_to_virt(apic->trampoline_phys_high)) =
@@ -23,6 +27,8 @@ static inline void smpboot_setup_warm_re
 
 static inline void smpboot_restore_warm_reset_vector(void)
 {
+	unsigned long flags;
+
 	/*
 	 * Install writable page 0 entry to set BIOS data area.
 	 */
@@ -32,7 +38,9 @@ static inline void smpboot_restore_warm_
 	 * Paranoid:  Set warm reset code and vector here back
 	 * to default values.
 	 */
+	spin_lock_irqsave(&rtc_lock, flags);
 	CMOS_WRITE(0, 0xf);
+	spin_unlock_irqrestore(&rtc_lock, flags);
 
 	*((volatile u32 *)phys_to_virt(apic->trampoline_phys_low)) = 0;
 }