[Python-Dev] Bugs in thread_nt.h (original) (raw)

Sturla Molden sturla at molden.no
Thu Mar 10 12🔞22 CET 2011

Den 10.03.2011 11:06, skrev Scott Dial:

http://www.kernel.org/doc/Documentation/volatile-considered-harmful.txt

The important part here (forgive me for being a pedant) is that register allocation of the (1) 'owned' field is actually unwanted, and (2) Microsoft specify 'volatile' in calls to the Interlocked* API-functions.

I am sorry for misreading 32 bits (4 bytes) as 32 bytes. That is obviously very different. If Microsoft's malloc is sufficient, why does MSDN tell us to use _aligned_malloc instead of malloc?

The rest is a comment to the link and a bit OT:

Their argument is that (1) volatile supresses optimization; (2) within a critical section, access to shared data is synchronized; and thus (3) volatile is critical there.

That is true, to some extent.

Volatile is not a memory barrier, nor a mutex. Volatile's main purpose is to prevent the compiler from storing a variable in a register. Volatile might be used incorrectly if we don't understand this.

Obvious usecases for volatile are:

Even though volatile is not needed for processing within a critical section, we still need the shared data to be re-loaded upon entering and er-written upon leaving. We can use a typecast to non-volatile within a critical section, and achieve both data consisitency and compiler optimization. OpenMP has a better mechanism for this, where a flush-operand (#pragma omp flush) will force a synchronization of shared data among threads (write and reload), and volatile is never needed for consistency of shared data.

Sturla

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