Memory barriers in userspace? (Linux, x86-64)
It is easy to set memory barriers on the kernel side: the macros mb, wmb, rmb, etc. are always in place thanks to the Linux kernel headers. How to accomplish this on the user side? ...
memory-barriers
Memory barriers and large structs?
Let's say I've got a struct that consist of 100 bytes. What guarantees have I got about the following code? m_myLargeStruct = someValue; // copying 100 bytes Thread.MemoryBarrier(); // Executed by another thread, after "Thread.MemoryBarrier" was called by the first thread Console.WriteLine(m_myLargeStruct.ToString()); Does the memo...
How do I Understand Read Memory Barriers and Volatile
Some languages provide a volatile modifier that is described as performing a "read memory barrier" prior to reading the memory that backs a variable. A read memory barrier is commonly described as a way to ensure that the CPU has performed the reads requested before the barrier before it performs a read requested after the barrier. Howe...
How does _ReadWriteBarrier propagate up the call tree ?
I'm looking at this bit of text in the documentation for Visual C++'s _ReadWriteBarrier intrinsic: In past versions of the Visual C++ compiler, the _ReadWriteBarrier and _WriteBarrier functions were enforced only locally and did not affect functions up the call tree. In Visual C++ 2005 and later, these functions are enforce...
Java LockSupport Memory Consistency
Java 6 API question. Does calling LockSupport.unpark(thread) have a happens-before relationship to the return from LockSupport.park in the just-unparked thread? I strongly suspect the answer is yes, but the Javadoc doesn't seem to mention it explicitly. ...
How do JVM's implicit memory barriers behave when chaining constructors?
Referring to my earlier question on incompletely constructed objects, I have a second question. As Jon Skeet pointed out, there's an implicit memory barrier in the end of a constructor that makes sure that final fields are visible to all threads. But what if a constructor calls another constructor; is there such a memory barrier in the ...
Thread Synchronisation 101
Previously I've written some very simple multithreaded code, and I've always been aware that at any time there could be a context switch right in the middle of what I'm doing, so I've always guarded access the shared variables through a CCriticalSection class that enters the critical section on construction and leaves it on destruction. ...
Are memory barriers necessary for atomic reference counting shared immutable data?
I have some immutable data structures that I would like to manage using reference counts, sharing them across threads on an SMP system. Here's what the release code looks like: void avocado_release(struct avocado *p) { if (atomic_dec(p->refcount) == 0) { free(p->pit); free(p->juicy_innards); free(p); } }...
Compiler reordering around mutex boundaries?
Suppose I have my own non-inline functions LockMutex and UnlockMutex, which are using some proper mutex - such as boost - inside. How will the compiler know not to reorder other operations with regard to calls to the LockMutex and UnlockMutex? It can not possibly know how will I implement these functions in some other compilation unit. ...
Memory barriers vs. interlocked operations
I am trying to improve my understanding of memory barriers. Suppose we have a weak memory model and we adapt Dekker's algorithm. Is it possible to make it work correctly under the weak memory model by adding memory barriers? I think the answer is a surprising no. The reason (if I am correct) is that although a memory barrier can be used...
order and barrier:what is the equivalent instruction on x86 for 'lwsync' on PowerPC?
My code is simple as below.I found rmb and wmb for read and write,but found no general one.lwsync is available on PowerPC,but what is the replacement for x86?Thanks in advance. #define barrier() __asm__ volatile ("lwsync") ... lock() if(!pInst); { T* temp=new T; barrier(); pInst=temp; } unlock...
Why we need Thread.MemoryBarrier()?
In C#4 in a Nutshell, the author show that this class can write 0 sometimes without MemoryBarrier. I can´t reproduce in my Core2Duo: public class Foo { int _answer; bool _complete; public void A() { _answer = 123; //Thread.MemoryBarrier(); // Barrier 1 _complete = true; //Thread.MemoryB...
memory fences/barriers in C++: does boost or other libraries have them?
I am reading these days about memory fences and barriers as a way to synchronize multithreaded code and avoid code reordering. I usually develop in C++ under Linux OS and I use boost libs massively but I am not able to find any class related to it. Do you know if memory barrier of fences are present in boost or if there is a way to ach...