From: Richard Mortimer Date: Tue, 17 Jan 2006 23:21:01 +0000 (-0800) Subject: [SPARC64]: Eliminate race condition reading Hummingbird STICK register X-Git-Tag: v2.6.16-rc2~368^2~3 X-Git-Url: http://pilppa.com/gitweb/?a=commitdiff_plain;h=9eb3394bf2037120881a8846bc67064f49325366;p=linux-2.6-omap-h63xx.git [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 Signed-off-by: David S. Miller --- diff --git a/arch/sparc64/kernel/time.c b/arch/sparc64/kernel/time.c index 459c8fbe02b..a22930d62ad 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)