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Reorder assignments in cmn_systimer_get() to avoid race.

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Before, it was possible for the system timer counter to wrap from high to low
after its value had been read but before the value of cmn_systimer_counter
was read and so the resulting sum of those values would be too large.

By reordering the assignments, we can now detect if cmn_systimer_counter has
been advanced while reading the system timer counter value and, if so, repeat
the assignments to ensure consistent values.
This commit is contained in:
Mark Burton 2014-02-23 18:07:06 +00:00
parent e11180bc75
commit e9f23cc33f
1 changed files with 2 additions and 2 deletions
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4
src/common_tmr.c
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@ -425,12 +425,12 @@ timer_data_type cmn_systimer_get(void)
{
u64 tempsys, tempcnt, crtsys;
tempcnt = platform_timer_sys_raw_read();
tempsys = cmn_systimer_counter;
tempcnt = platform_timer_sys_raw_read();
while( ( crtsys = cmn_systimer_counter ) != tempsys )
{
tempcnt = platform_timer_sys_raw_read();
tempsys = crtsys;
tempcnt = platform_timer_sys_raw_read();
}
crtsys += tempcnt / cmn_systimer_ticks_for_us;
if( crtsys > PLATFORM_TIMER_SYS_MAX ) // timer overflow