Does Interlocked.CompareExchange usesuse a memory barrier?

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edited Oct 17, 2009 at 9:29

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I'm reading Joe Duffy's post about Volatile reads and writes, and timeliness, and i'm trying to understand something about the last code sample in the post:

while (Interlocked.CompareExchange(ref m_state, 1, 0) != 0) ; m_state = 0; while (Interlocked.CompareExchange(ref m_state, 1, 0) != 0) ; m_state = 0; …

When the second CMPXHGCMPXCHG operation is executed, does it use a memory barrier to ensure that the value of m_state is indeed the latest value written to it? Or will it just use some value that is already stored in the processor's cache? (assuming m_state isn't declared as volatile).
If I understand correctly, if CMPXHGCMPXCHG won't use a memory barrier, then the whole lock acquisition procedure won't be fair since it's highly likely that the thread that was the first to acquire the lock, will be the one that will acquire all of following locks. Did I understand correctly, or am I missing out on something here?

Edit: The main question is actually whether calling to CompareExchange will cause a memory barrier before attempting to read m_state's value. So whether assigning 0 will be visible to all of the threads when they try to call CompareExchange again.

I'm reading Joe Duffy's post about Volatile reads and writes, and timeliness, and i'm trying to understand something about the last code sample in the post:

while (Interlocked.CompareExchange(ref m_state, 1, 0) != 0) ; m_state = 0; while (Interlocked.CompareExchange(ref m_state, 1, 0) != 0) ; m_state = 0; …

When the second CMPXHG operation is executed, does it use a memory barrier to ensure that the value of m_state is indeed the latest value written to it? Or will it just use some value that is already stored in the processor's cache? (assuming m_state isn't declared as volatile).
If I understand correctly, if CMPXHG won't use a memory barrier, then the whole lock acquisition procedure won't be fair since it's highly likely that the thread that was the first to acquire the lock, will be the one that will acquire all of following locks. Did I understand correctly, or am I missing out on something here?

Edit: The main question is actually whether calling to CompareExchange will cause a memory barrier before attempting to read m_state's value. So whether assigning 0 will be visible to all of the threads when they try to call CompareExchange again.

I'm reading Joe Duffy's post about Volatile reads and writes, and timeliness, and i'm trying to understand something about the last code sample in the post:

while (Interlocked.CompareExchange(ref m_state, 1, 0) != 0) ; m_state = 0; while (Interlocked.CompareExchange(ref m_state, 1, 0) != 0) ; m_state = 0; …

When the second CMPXCHG operation is executed, does it use a memory barrier to ensure that the value of m_state is indeed the latest value written to it? Or will it just use some value that is already stored in the processor's cache? (assuming m_state isn't declared as volatile).
If I understand correctly, if CMPXCHG won't use a memory barrier, then the whole lock acquisition procedure won't be fair since it's highly likely that the thread that was the first to acquire the lock, will be the one that will acquire all of following locks. Did I understand correctly, or am I missing out on something here?

Edit: The main question is actually whether calling to CompareExchange will cause a memory barrier before attempting to read m_state's value. So whether assigning 0 will be visible to all of the threads when they try to call CompareExchange again.