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[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>
This commit is contained in:
Richard Mortimer 2006-01-17 15:21:01 -08:00 committed by David S. Miller
parent 2664b25051
commit 9eb3394bf2
1 changed files with 11 additions and 11 deletions
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22
arch/sparc64/kernel/time.c
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@ -280,9 +280,9 @@ static struct sparc64_tick_ops stick_operations __read_mostly = {
* Since STICK is constantly updating, we have to access it carefully.
*
* The sequence we use to read is:
* 1) read low
* 2) read high
* 3) read low again, if it rolled over increment high by 1
* 1) read high
* 2) read low
* 3) read high again, if it rolled re-read both low and high again.
*
* Writing STICK safely is also tricky:
* 1) write low to zero
@ -295,18 +295,18 @@ static struct sparc64_tick_ops stick_operations __read_mostly = {
static unsigned long __hbird_read_stick(void)
{
unsigned long ret, tmp1, tmp2, tmp3;
unsigned long addr = HBIRD_STICK_ADDR;
unsigned long addr = HBIRD_STICK_ADDR+8;
__asm__ __volatile__("ldxa [%1] %5, %2\n\t"
"add %1, 0x8, %1\n\t"
"ldxa [%1] %5, %3\n\t"
__asm__ __volatile__("ldxa [%1] %5, %2\n"
"1:\n\t"
"sub %1, 0x8, %1\n\t"
"ldxa [%1] %5, %3\n\t"
"add %1, 0x8, %1\n\t"
"ldxa [%1] %5, %4\n\t"
"cmp %4, %2\n\t"
"blu,a,pn %%xcc, 1f\n\t"
" add %3, 1, %3\n"
"1:\n\t"
"sllx %3, 32, %3\n\t"
"bne,a,pn %%xcc, 1b\n\t"
" mov %4, %2\n\t"
"sllx %4, 32, %4\n\t"
"or %3, %4, %0\n\t"
: "=&r" (ret), "=&r" (addr),
"=&r" (tmp1), "=&r" (tmp2), "=&r" (tmp3)