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  1. 18 1月, 2006 1 次提交
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      [SPARC64]: Eliminate race condition reading Hummingbird STICK register · 9eb3394b
      由 Richard Mortimer 提交于 
      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: NRichard Mortimer <richm@oldelvet.org.uk>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
      9eb3394b
  3. 17 1月, 2006 39 次提交