Problems with semaphores, mutexes, and atomic?

[Dave Hylands]

Hi Russell,

On Mon, Jun 20, 2011 at 1:27 AM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> 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.

Thank you. My sanity has been restored.

I added a down on thread_wait in the init routine, and now all of test
cases pass for all of the variations (CONFIG_SMP=n, CONFIG_SMP=y, with
1 or 2 processors).

Now I need to dig deeper to find the real reason why my original code,
which prompted me to go down this path is failing.

-- 
Dave Hylands
Shuswap, BC, Canada
http://www.davehylands.com