On 06/27/2011 08:03 PM, Andrew Brunner wrote:
entercricitalsection();
loop
a:=b+c;
end loop;
leavecriticalsection();
thread 2
can read a and b and c at any state. If you want an accurate view of
a,b,c you need to employ interlocked statements :-)
Hmm.
In this example, b and c are not modified so in fact any view is
accurate at a certain point of time. Moreover doing the critical section
outside of a lop does not make much sense.
A more obvious example would be
Thread 1:
loop
entercricitalsection();
a:=a+1;
b:=b+a;
leavecriticalsection();
end loop;
Here another thread just fetching a and b could get inconsistent (not
representing a the same loop) value of a and b.
IMHO if the other thread would do
entercricitalsection();
a1:=a;
a2:=b+a;
leavecriticalsection();
write a1 and a2
it will show consistant values.
Here, it is not possible to force consistent values by interlocked
instructions, as there is no interlocked instruction that does a:=a+1
and b:=b+a in a single hardware lock. So using the critical section in
both threads is necessary to force consistency.
An example that can be done with interlocked instructions (and thus
prevents the huge overhead introduced by a critical section) is
Thread 1:
loop
a:=a+1;
end loop;
Thread 2:
loop
a:=a-1;
end loop;
-Michael
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