CODEWITHSUNDEEP
c++multithreadingconcurrencyatomic
MRB
MRB

Reputation: 3822

example about memory ordering in c++

Consider following example:

 -Thread 1-
 y.store (20, memory_order_relaxed);
 x.store (10, memory_order_release);

 -Thread 2-
 if (x.load(memory_order_acquire) == 10) {
     assert (y.load(memory_order_relaxed) == 20)
     y.store (10, memory_order_release)
 }

 -Thread 3-
 if (y.load(memory_order_acquire) == 10)
     assert (x.load(memory_order_relaxed) == 10)

In this example second assert will fire(am i correct?). is it because there is no store to x in thread 2 before y.store (10, memory_order_release)?

(in cppreference.com they say this sentence about release: "A store operation with this memory order performs the release operation: prior writes to other memory locations become visible to the threads that do a consume or an acquire on the same location.")

Can i change the order of store to y in thread2 from release to sec/cst to solve the problem?

Upvotes: 0

Views: 130

Answers (1)

briand
briand

Reputation: 300

Your example isn't complete because you haven't specified initial values for x & y. But let's assume that the thread that starts all threads has initialized both to 0.

Then if thread 2 does a store to y, it must have read from thread 1's store to x and synchronized with it. If thread 3's load from y reads thread 2's store to y, it must synchronize also. Therefore, the store to x in thread 1 must happen before the load in thread 3 and it must happen after the initialization store to x. Thus thread 3's x.load must get the value of 10. Happens before in the absence of consume is transitive.

I suggest using CDSChecker on these examples to see what values are possible.

Upvotes: 2

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