On 09/18/12 13:21, Stefan Teleman wrote:
On Tue, Sep 18, 2012 at 12:43 PM, Liviu Nicoara <[email protected]> wrote:
I am attaching a test program which, while 100% MT-safe, is flagged by
the Solaris thread analyzer.
The program as written is not thread safe. It is reading the value of
the counter variable and performing a zero comparison outside of a
mutex lock:
Stefan, I urge you to consider the argument on its merits. Yes, the thread
analyzer flags it, but it is nonetheless MT-safe. Specifically:
1. writes are properly synchronized wrt themselves
2. reads are inherently thread-safe wrt themselves
3. reads are properly synchronized wrt writes
4.no thread can possibly observe an intermediate or otherwise incomplete value.
I will also add that the flag is either 0 or 1 during the execution of the
program, with only one transition from 0 to 1, performed by one single thread.
I will concede that I might be wrong and I am open to arguments. I would accept
as a counter-argument this program if you can show a runtime failure. I would
also accept as argument a scenario under which two threads would see
inconsistent/incorrect values or write the variable more than once, etc.
Thanks,
Liviu
for (size_t i = 0; i < nloops; ++i) {
if (counter == 0) { // <----------- !!!!
pthread_mutex_lock (&lock);
if (counter == 0)
++counter;
pthread_mutex_unlock (&lock);
}
else {
// counter value is safe to use here
}
}
