Comment #19 on issue 18488 by timurrrr: Possible data race on
HistoryAddPageArgs
http://code.google.com/p/chromium/issues/detail?id=18488
Consider two threads running the same code (counter is a shared variable,
initially
set to 0):
void ThreadProc() {
for (int i = 0; i < 100000; i++)
counter++;
}
experiments show that counter becomes 19XXXX, not 200000.
Now consider a data race on reference counter.
If two increments or decrements happen simultaneously, the reference
counter may
become +1 or -1 to the correct value.
In case of "+1", we are unlikely to achieve zero refcount at all, so we are
likely to
have a memory leak.
In case of "-1", reference counter may become zero more than once; and it
can also
become -1.
For example, consider the following interleaving (T1, T2 = thread{1,2})
T1|T2| refcount
XXXXX| 0
++| | 1
++|++|!!2, not 3!!
--| | 1, not 2
|--| 0, not 1 -> free is called
++| | 1, not 2
--| | 0, not 1 -> free is called the second time
--| | -1, not 0
Please note that there is an equal number of "++" and "--" in this
interleaving
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