I have a dtrace variable which is a counter which I'm incrementing on
entry to a kernel function, and decrementing on function return, so I
expect the value to be the number of threads which are currently in that
function. However, the value slowly climbs to values way higher than the
number of threads I could possible have in the function. It's rather
like what I'd expect if I was manipulating a counter in C from lots of
threads without protection from a mutex, and indeed if I turn off all
except 1 CPU, the counter seems to work perfectly and give me the values
I'd expect.
So, is there anyway to safely have multiple threads increment and
decrement a counter? I guess I could use an array indexed by cpu, but I
can't think how to add together all the array elements to get the total,
which I'm using in a tick probe.
--
Andrew
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