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[POWERPC] Fix gettimeofday inaccuracies

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There are two problems in the powerpc gettimeofday code which can
cause incorrect results to be returned.

The first is that there is a race between do_gettimeofday and the
timer interrupt:

1. do_gettimeofday does get_tb()

2. decrementer exception on boot cpu which runs timer_recalc_offset,
   which also samples the timebase and updates the do_gtod structure
   with a greater timebase value.

3. do_gettimeofday calls __do_gettimeofday, which leads to the
   negative result from tb_val - temp_varp->tb_orig_stamp.

The second is caused by taking the boot cpu offline, which can cause
the value of tb_last_jiffy to be increased past the currently
available timebase, causing the same underflow as above.

[paulus@samba.org - define and use data_barrier() instead of mb().]

Signed-off-by: Nathan Lynch <ntl@pobox.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
This commit is contained in:
Nathan Lynch 2006-08-22 20:36:05 -05:00 committed by Paul Mackerras
parent aa74a30be9
commit 5db9fa9593
2 changed files with 26 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

25
arch/powerpc/kernel/time.c
View File

@ -417,7 +417,7 @@ static __inline__ void timer_check_rtc(void)
/*
* This version of gettimeofday has microsecond resolution.
*/
static inline void __do_gettimeofday(struct timeval *tv, u64 tb_val)
static inline void __do_gettimeofday(struct timeval *tv)
{
unsigned long sec, usec;
u64 tb_ticks, xsec;
@ -431,7 +431,12 @@ static inline void __do_gettimeofday(struct timeval *tv, u64 tb_val)
* without a divide (and in fact, without a multiply)
*/
temp_varp = do_gtod.varp;
tb_ticks = tb_val - temp_varp->tb_orig_stamp;
/* Sampling the time base must be done after loading
* do_gtod.varp in order to avoid racing with update_gtod.
*/
data_barrier(temp_varp);
tb_ticks = get_tb() - temp_varp->tb_orig_stamp;
temp_tb_to_xs = temp_varp->tb_to_xs;
temp_stamp_xsec = temp_varp->stamp_xsec;
xsec = temp_stamp_xsec + mulhdu(tb_ticks, temp_tb_to_xs);
@ -464,7 +469,7 @@ void do_gettimeofday(struct timeval *tv)
tv->tv_usec = usec;
return;
}
__do_gettimeofday(tv, get_tb());
__do_gettimeofday(tv);
}
EXPORT_SYMBOL(do_gettimeofday);
@ -650,6 +655,7 @@ void timer_interrupt(struct pt_regs * regs)
int next_dec;
int cpu = smp_processor_id();
unsigned long ticks;
u64 tb_next_jiffy;
#ifdef CONFIG_PPC32
if (atomic_read(&ppc_n_lost_interrupts) != 0)
@ -691,11 +697,14 @@ void timer_interrupt(struct pt_regs * regs)
continue;
write_seqlock(&xtime_lock);
tb_last_jiffy += tb_ticks_per_jiffy;
tb_last_stamp = per_cpu(last_jiffy, cpu);
do_timer(regs);
timer_recalc_offset(tb_last_jiffy);
timer_check_rtc();
tb_next_jiffy = tb_last_jiffy + tb_ticks_per_jiffy;
if (per_cpu(last_jiffy, cpu) >= tb_next_jiffy) {
tb_last_jiffy = tb_next_jiffy;
tb_last_stamp = per_cpu(last_jiffy, cpu);
do_timer(regs);
timer_recalc_offset(tb_last_jiffy);
timer_check_rtc();
}
write_sequnlock(&xtime_lock);
}

9
include/asm-powerpc/system.h
View File

@ -53,6 +53,15 @@
#define smp_read_barrier_depends() do { } while(0)
#endif /* CONFIG_SMP */
/*
* This is a barrier which prevents following instructions from being
* started until the value of the argument x is known. For example, if
* x is a variable loaded from memory, this prevents following
* instructions from being executed until the load has been performed.
*/
#define data_barrier(x) \
asm volatile("twi 0,%0,0; isync" : : "r" (x) : "memory");
struct task_struct;
struct pt_regs;