Here's another exercise to understand barriers. Once again, please
review to see if this is indeed correct.
This patch removes the volatile keyword for cmos_lock and tries to make
the code correct using barriers.
-- Steve
Signed-off-by: Steven Rostedt <[email protected]>
Index: linux-2.6.18-rc1/include/asm-i386/mc146818rtc.h
===================================================================
--- linux-2.6.18-rc1.orig/include/asm-i386/mc146818rtc.h 2006-07-14 09:09:14.000000000 -0400
+++ linux-2.6.18-rc1/include/asm-i386/mc146818rtc.h 2006-07-14 09:22:50.000000000 -0400
@@ -31,7 +31,7 @@
* atomically claim the lock and set the owner.
*/
#include <linux/smp.h>
-extern volatile unsigned long cmos_lock;
+extern unsigned long cmos_lock;
/*
* All of these below must be called with interrupts off, preempt
@@ -43,8 +43,10 @@ static inline void lock_cmos(unsigned ch
unsigned long new;
new = ((smp_processor_id()+1) << 8) | reg;
for (;;) {
- if (cmos_lock)
+ if (cmos_lock) {
+ cpu_relax();
continue;
+ }
if (__cmpxchg(&cmos_lock, 0, new, sizeof(cmos_lock)) == 0)
return;
}
@@ -52,14 +54,16 @@ static inline void lock_cmos(unsigned ch
static inline void unlock_cmos(void)
{
- cmos_lock = 0;
+ set_wmb(cmos_lock, 0);
}
static inline int do_i_have_lock_cmos(void)
{
+ barrier();
return (cmos_lock >> 8) == (smp_processor_id()+1);
}
static inline unsigned char current_lock_cmos_reg(void)
{
+ barrier();
return cmos_lock & 0xff;
}
#define lock_cmos_prefix(reg) \
Index: linux-2.6.18-rc1/arch/i386/kernel/time.c
===================================================================
--- linux-2.6.18-rc1.orig/arch/i386/kernel/time.c 2006-07-14 09:08:56.000000000 -0400
+++ linux-2.6.18-rc1/arch/i386/kernel/time.c 2006-07-14 09:24:06.000000000 -0400
@@ -86,7 +86,7 @@ EXPORT_SYMBOL(rtc_lock);
* register we are working with. It is required for NMI access to the
* CMOS/RTC registers. See include/asm-i386/mc146818rtc.h for details.
*/
-volatile unsigned long cmos_lock = 0;
+unsigned long cmos_lock = 0;
EXPORT_SYMBOL(cmos_lock);
/* Routines for accessing the CMOS RAM/RTC. */
-
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