diff options
author | Richard Mortimer <richm@oldelvet.org.uk> | 2006-01-17 15:21:01 -0800 |
---|---|---|
committer | David S. Miller <davem@davemloft.net> | 2006-01-17 15:21:01 -0800 |
commit | 9eb3394bf2037120881a8846bc67064f49325366 (patch) | |
tree | 6782663f5b5a13cf8f98c4341637322650b42f9a /arch | |
parent | 2664b25051f7ab96b22b199aa2f5ef6a949a4296 (diff) |
[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>
Diffstat (limited to 'arch')
-rw-r--r-- | arch/sparc64/kernel/time.c | 22 |
1 files changed, 11 insertions, 11 deletions
diff --git a/arch/sparc64/kernel/time.c b/arch/sparc64/kernel/time.c index 459c8fbe02b4..a22930d62adf 100644 --- a/arch/sparc64/kernel/time.c +++ b/arch/sparc64/kernel/time.c @@ -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) |