On Wednesday 20 April 2005 04:39 pm, John Giacomoni wrote:
can someone give me an example of a situation where one needs to use memory barriers to ensure "correctness" when doing writes as above?
One basic example:
struct foo *p = malloc(sizeof(struct foo)); // Step 1 p->bar = 0; // Step 2 atomic(global_p = p); // Step 3
This is a pretty common idiom: Allocate a new data structure, initialize it, then assign the pointer to a shared global value so that the fully-initialized structure becomes visible all at once.
Many people miss an important detail: A barrier is needed just before the final pointer assignment. Otherwise, the pointer might reach shared memory (Step 3) before the initialization in Step 2 happens. (This can happen because of compiler reordering or processor write caching.)
Variations of this plagued the early attempts to correctly define Java's threading primitives. (It was possible to crash a lot of early JVMs by creating new objects in one thread and accessing them in another; every now and then the accessing thread would see an uninitialized vtable and crash the entire JVM.)
Tim Kientzle
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