[SPARC64]: Eliminate race condition reading Hummingbird STICK register
Ensure a consistent value is read from the STICK register by ensuring that both high and low are read without high changing due to a roll over of the low register. Various Debian/SPARC users (myself include) have noticed problems with Hummingbird based systems. The symptoms are that the system time is seen to jump forward 3 days, 6 hours, 11 minutes give or take a few seconds. In many cases the system then hangs some time afterwards. I've spotted a race condition in the code to read the STICK register. I could not work out why 3d, 6h, 11m is important but guess that it is due to the 2^32 jump of STICK (forwards on one read and then the next read will seem to be backwards) during a timer interrupt. I'm guessing that a change of -2^32 will get converted to a large unsigned increment after the arithmetic manipulation between STICK, nanoseconds, jiffies etc. I did a test where I modified __hbird_read_stick to artificially inject rollover faults forcefully every few seconds. With this I saw the clock jump over 6 times in 12 hours compared to once every month or so. Signed-off-by: Richard Mortimer <richm@oldelvet.org.uk> Signed-off-by: David S. Miller <davem@davemloft.net>
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@ -280,9 +280,9 @@ static struct sparc64_tick_ops stick_operations __read_mostly = {
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* Since STICK is constantly updating, we have to access it carefully.
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*
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* The sequence we use to read is:
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* 1) read low
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* 2) read high
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* 3) read low again, if it rolled over increment high by 1
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* 1) read high
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* 2) read low
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* 3) read high again, if it rolled re-read both low and high again.
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*
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* Writing STICK safely is also tricky:
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* 1) write low to zero
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@ -295,18 +295,18 @@ static struct sparc64_tick_ops stick_operations __read_mostly = {
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static unsigned long __hbird_read_stick(void)
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{
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unsigned long ret, tmp1, tmp2, tmp3;
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unsigned long addr = HBIRD_STICK_ADDR;
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unsigned long addr = HBIRD_STICK_ADDR+8;
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__asm__ __volatile__("ldxa [%1] %5, %2\n\t"
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"add %1, 0x8, %1\n\t"
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"ldxa [%1] %5, %3\n\t"
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__asm__ __volatile__("ldxa [%1] %5, %2\n"
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"1:\n\t"
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"sub %1, 0x8, %1\n\t"
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"ldxa [%1] %5, %3\n\t"
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"add %1, 0x8, %1\n\t"
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"ldxa [%1] %5, %4\n\t"
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"cmp %4, %2\n\t"
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"blu,a,pn %%xcc, 1f\n\t"
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" add %3, 1, %3\n"
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"1:\n\t"
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"sllx %3, 32, %3\n\t"
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"bne,a,pn %%xcc, 1b\n\t"
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" mov %4, %2\n\t"
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"sllx %4, 32, %4\n\t"
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"or %3, %4, %0\n\t"
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: "=&r" (ret), "=&r" (addr),
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"=&r" (tmp1), "=&r" (tmp2), "=&r" (tmp3)
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