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Subject[PATCH] get rid of ARCH_HAVE_XTIME_LOCK
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As suggested by Andrew, we can use __attribute__((weak)) to get rid of 
ARCH_HAVE_XTIME_LOCK

Please note I compiled, and boot tested on ia32 this patch, and it seems OK.
I compiled on x86_64 and got same resulting vmlinux image.

But I suspect some tools might have problems because vmlinux have its
first 'weak data symbol' defined. AFAIK __attribute__((weak)) was only used
on text symbols.

# nm vmlinux | grep ' V '
c03b01c0 V xtime_lock
[PATCH] get rid of ARCH_HAVE_XTIME_LOCK

ARCH_HAVE_XTIME_LOCK is used by x86_64 arch . This arch needs to place a read
only copy of xtime_lock into vsyscall page. This read only copy is named
__xtime_lock, and xtime_lock is defined in arch/x86_64/kernel/vmlinux.lds.S
as an alias. So the declaration of xtime_lock in kernel/timer.c was guarded
by ARCH_HAVE_XTIME_LOCK define, defined to true on x86_64.

We can get same result with _attribute__((weak)) in the declaration. linker
should do the job.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
--- linux-2.6.19/kernel/timer.c 2006-12-11 11:25:50.000000000 +0100
+++ linux-2.6.19-ed/kernel/timer.c 2006-12-11 11:31:30.000000000 +0100
@@ -1020,11 +1020,9 @@ static inline void calc_load(unsigned lo
* This read-write spinlock protects us from races in SMP while
* playing with xtime and avenrun.
*/
-#ifndef ARCH_HAVE_XTIME_LOCK
-__cacheline_aligned_in_smp DEFINE_SEQLOCK(xtime_lock);
+__attribute__((weak)) __cacheline_aligned_in_smp DEFINE_SEQLOCK(xtime_lock);

EXPORT_SYMBOL(xtime_lock);
-#endif

/*
* This function runs timers and the timer-tq in bottom half context.
--- linux-2.6.19/include/linux/time.h 2006-12-11 11:34:54.000000000 +0100
+++ linux-2.6.19-ed/include/linux/time.h 2006-12-11 11:34:54.000000000 +0100
@@ -90,7 +90,7 @@ static inline struct timespec timespec_s

extern struct timespec xtime;
extern struct timespec wall_to_monotonic;
-extern seqlock_t xtime_lock;
+extern seqlock_t xtime_lock __attribute__((weak));

void timekeeping_init(void);

--- linux-2.6.19/include/asm-x86_64/vsyscall.h 2006-12-11 11:34:54.000000000 +0100
+++ linux-2.6.19-ed/include/asm-x86_64/vsyscall.h 2006-12-11 11:34:54.000000000 +0100
@@ -55,11 +55,6 @@ extern struct vxtime_data vxtime;
extern int vgetcpu_mode;
extern struct timezone sys_tz;
extern int sysctl_vsyscall;
-extern seqlock_t xtime_lock;
-
-extern int sysctl_vsyscall;
-
-#define ARCH_HAVE_XTIME_LOCK 1

#endif /* __KERNEL__ */
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