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Use Critical Sections (Preferably Locks) to Eliminate Races

Code Must Never Move Out

Let's see what "code can't move out" means in the context of the following code, which acquires a mutex mut that protects two integers x and y:

mut.lock();   // enter ("acquire")      
              // critical section
x = 42;      // where can this line
             // appear to move to?
y = 43;          
mut.unlock(); // exit ("release")
              //critical section

What are the legal reorderings of the assignment to x? It is perfectly legal for the system to transform the above code to:

y = 43;
x = 42;   // ok: can move down
          // past y's assignment

because both assignments still happen inside the protected critical section (and do not otherwise depend on each other, assuming x and y are independent and not aliased). A system may not, however, transform the code to either

x = 42;    // invalid: race bait
y = 43;


y = 43;
x = 42;    // invalid: race bait

because either of these would move the assignment outside the critical section and therefore create a potential race on x.

So what about moving code into a critical section?

It's Okay For Code to Move In

Consider an adapted example:

x = "life";   // where can this line
              // appear to move to?
mut.lock();   // enter ("acquire")
              // critical section
y = "universe";
mut.unlock(); // exit ("release") 
              // critical section
z = "everything"; // where can this 
                 // line appear to
                 // move to?

What are the legal reorderings of the assignments to x and z? Again assuming that x, y, and z are independent and not aliased, it is perfectly legal for the system to transform the above code to:

z = "everything"; // ok: can move as
                  // far up as this
y = "universe";
x = "life";      // ok: can move as
                 // far down as this

Even though the moved lines now run while holding the lock, it doesn't alter the meaning or correctness of the code. It is always safe to add extra guarantees; in this case, to hold a lock a little longer.

But it is not safe to arbitrarily remove guarantees, such as to fail to hold a needed lock. Therefore, a system cannot arbitrarily reorder the code to cross either fence the wrong way:

z = "everything"; // invalid: race bait
y = "universe";
x = "life";     // invalid: race bait

Note that this is true even though the assignments to x and z were not initially inside the critical section. For example, what if setting y = "universe" is treated as a flag that tells another thread that x is now ready to be shared, so that y publishes x? That is why no code must pass the release fence in the downward direction. [3]

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