Problems with semaphores, mutexes, and atomic?

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Sun, Jun 19, 2011 at 11:39:49PM -0700, Dave Hylands wrote:
> Each thread performs the loop a number of times, and at the end the
> count often doesn't have the expected value if the locks were in fact
> atomic.
> 
> Typical output looks like:
> 
> 192.168.0.50  ~ # modprobe test-mutex
> [   26.220000] Testing mutex...
> [   26.230000]     Launching threads ..........
> [   26.300000]     Waiting for threads to finish ..........
> [   26.310000]   counter = 999888 FAIL
> [   26.310000] Testing semaphore...
> [   26.310000]     Launching threads ..........
> [   26.440000]     Waiting for threads to finish ..........
> [   27.810000]   counter = 999575 FAIL
> [   27.810000] Testing atomic...
> [   27.820000]     Launching threads ..........
> [   28.030000]     Waiting for threads to finish ..........
> [   28.040000]  counter = 9910929 FAIL

The answer is quite simple.  You're not waiting for all the threads to
finish.  The first down(&thread_wait) will succeed, so the values you're
getting above could be up to 100000 (NUM_ATOMIC_ITER) out.  And they
are.