Commit 5db9fa95 authored by Nathan Lynch's avatar Nathan Lynch Committed by Paul Mackerras
Browse files

[POWERPC] Fix gettimeofday inaccuracies 



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: default avatarNathan Lynch <ntl@pobox.com>
Signed-off-by: default avatarPaul Mackerras <paulus@samba.org>
parent aa74a30b

arch/powerpc/kernel/time.c

+17 −8
Original line number Diff line number Diff line
@@ -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_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);

	}

include/asm-powerpc/system.h

+9 −0
Original line number Diff line number Diff line
@@ -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;

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