When programming with threads, there are three very common mistakes that programmers often
make:
1. locking twice (depending on the system and type of lock, can cause crashes, hangs, or do bizarre things);
2. locking and not unlocking (i.e. failure to unlock);
3. deadlock (see next lecture).
4. Priority inversion - This is not an error per se, but an important issue that occurs Of these problems, locking twice is probably the easiest type of error to detect. Here's one example:
function f() { function g() {
lock(L); lock(L);
g(); // access shared data
unlock(L); unlock(L);
} }
So-called "recursive" locks can deal with this situation correctly, though normal locks will cause this thread to wait forever when the function g(), when called fromf(), then calls lock(L) on a previously-held lock. Dealing with this can lead to a common code pattern with certain functions designed only to be called with locks held:
function f(){
function g() {
function g_internal() {
lock(L); lock(L); // locks must be held here!
g_internal(); g_internal(); // access shared data
unlock(L); unlock(L); }
} }
Failure to unlock is slightly more dif?cult to detect. It can occur, for example, if the programmer forgets to release the lock in one of the possible execution branches of the function:
function f() {
lock();
if (x==0) {
// should also unlock here before returning!
return;
}
// do something
unlock();
return;
}
One way to deal with this is just to remember to unlock() at each possible return. Another is to have every return path go through the same section of code (and in C, goto is sometimes useful for this, despite its bad reputation).