Messages in this thread Patch in this message | | | Date | Fri, 27 Jan 2006 18:21:12 -0800 | From | Greg KH <> | Subject | [patch 07/12] Fix timekeeping on sparc64 ultra-IIe machines |
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2.6.15.2 -stable review patch. If anyone has any objections, please let us know.
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From: Richard Mortimer <richm@oldelvet.org.uk>
[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> Signed-off-by: Chris Wright <chrisw@sous-sol.org> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de> --- arch/sparc64/kernel/time.c | 22 +++++++++++----------- 1 file changed, 11 insertions(+), 11 deletions(-)
--- linux-2.6.15.1.orig/arch/sparc64/kernel/time.c +++ linux-2.6.15.1/arch/sparc64/kernel/time.c @@ -280,9 +280,9 @@ static struct sparc64_tick_ops stick_ope * 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_ope 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) -- - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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