Re: [PATCH/RFC] mingw: implement PTHREAD_MUTEX_INITIALIZER
- From
Erik Faye-Lund <kusmabite@gmail.com>
- Date
- Oct 25, 2011, 15:42 UTC
- Message-ID
- <CABPQNSZ8wesy-px-n1LYbVwFT3gBNcrHfe+_553sinTferqsog@mail.gmail.com>
- In-Reply-To
- <4EA6D594.90402@viscovery.net>
On Tue, Oct 25, 2011 at 5:28 PM, Johannes Sixt <j.sixt@viscovery.net> wrote:
Show 16 quoted lines
> Am 10/25/2011 16:55, schrieb Erik Faye-Lund:
>> +int pthread_mutex_lock(pthread_mutex_t *mutex)
>> +{
>> + if (mutex->autoinit) {
>> + if (InterlockedCompareExchange(&mutex->autoinit, -1, 1) != -1) {
>> + pthread_mutex_init(mutex, NULL);
>> + mutex->autoinit = 0;
>> + } else
>> + while (mutex->autoinit != 0)
>> + ; /* wait for other thread */
>> + }
>
> The double-checked locking idiom. Very suspicious. Can you explain why it
> works in this case? Why are no Interlocked functions needed for the other
> accesses of autoinit? ("It is volatile" is the wrong answer to this last
> question, BTW.)I agree that it should look a bit suspicious; I'm generally skeptical whenever I see 'volatile' in threading-code myself. But I think it's the right answer in this case. "volatile" means that the compiler cannot optimize away accesses, which is sufficient in this case.
Basically, the thread that gets the original 1 returned from InterlockedCompareExchange is the only one who writes to mutex->autoinit. All other threads only read the value, and the volatile should make sure they actually do. Since all 32-bit reads and writes are atomic on Windows (see http://msdn.microsoft.com/en-us/library/windows/desktop/ms684122(v=vs.85).aspx "Simple reads and writes to properly-aligned 32-bit variables are atomic operations.") and mutex->autoinit is a LONG, this should be safe AFAICT. In fact, Windows specifically does not have any explicitly atomic writes exactly for this reason.
The only ways mutex->autoinit can be updated is: - InterlockedCompareExchange compares it to 1, finds it's identical and inserts -1 - intialization is done Both these updates happens from the same thread.
Yes, details like this should probably go into the commit message ;)
If there's something I'm missing, please let me know.